The names of O. Hertwig, H. Fol, E. Selenka, W. Roux, H. E. Ziegler, E. B. Wilson, and E. G. Conklin are associated with the classic observations on the migration of the male and female pronuclei in the inseminated egg. These workers advanced several interpretations to account for the migration. Later R. Chambers (1917) presented evidence that the aster is a gelated body and that the astral centre is the centre of centripetally directed cytoplasmic currents. On the basis of this interpretation of the aster, a reinvestigation was made of the movements of the pronuclei.

A detailed study was made at the Tortugas Laboratory1 during the summer of 1937 on the remarkably transparent eggs of Lytechinus variegatus in which the egg nucleus usually lies near the periphery of the mature unfertilized egg for at least 3 hr. after the eggs are normally shed. The egg nucleus and sperm aster are readily visible and can be followed continuously throughout their movements from the periphery to the approximate centre of the egg. The sperm aster was observed, rather than the sperm nucleus, because the latter could be made sufficiently visible only by unduly compressing the egg. Compression of the egg showed that the sperm nucleus lies in or near the astral lake.2

Observations were also made on the eggs of Arbacia punctulata and Echinarachnius parma during the following summer at Wood’s Hole and at the Mount Desert Island Biological Laboratory. They showed that the movements of-the pronuclei in these two species are similar to those in the egg of Lytechinus.

The eggs were inseminated in finger-bowls and transferred within 1 min. to a hanging-drop of sea water on a cover-slip mounted over a deep depression slide, the cover-slip being sealed to the slide with vaseline. Eggs left in the drop underwent normal cleavage at least to the four-celled stage. Observations on the movement of the egg nucleus and of the sperm aster were completed within the first 15 min. By this time the egg nucleus had always moved into the lake.

Records of the positions of the egg nucleus and of the sperm aster were made simultaneously every 30 sec., the observations being made with an oil-immersion objective. The records were made by marking on paper the centres of the egg nucleus and astral lake, using a camera-lucida. The paths of the two bodies were then plotted from the points obtained. In order to obtain accurate plots the observations were made only on eggs in which the egg nucleus and the sperm aster were in the same focus and in focus with the periphery of the maximum diameter of the egg. These observations were discarded in which the egg nucleus and sperm aster moved appreciably out of focus.

The fertilized eggs of Lytechinus are usually spherical, but many may be aspherical having one or more flattened surfaces. A few minutes after fertilization, small, temporary, wave-like irregularities appear over the entire surface of all the eggs. Usually these irregularities were too minute to be taken into account, but all observations were discarded in which the contour of the egg showed an appreciable change while the movements of the two bodies were being followed. Care was taken in every case to prevent deformation of the eggs by pressure in the hanging-drop.

After the elimination of a large number of observations not in accordance with the conditions mentioned above, thirty reliable records in all were obtained.

(1) The movement of the sperm aster

As is well known for the echinoderm egg, the spermatozoon enters the egg at any point on its circumference. An exudation cone rises from the surface of the egg, thereby marking the point of entry for many minutes. The sperm head moves in a straight line at right angles to the surface along a narrow path of hyaline cytoplasm extending a short distance from the periphery. As the sperm head moves further inward, it disappears in the granular cytoplasm where the diminutive sperm aster appears a few moments later (usually 2-3 min. after the penetration of the spermatozoon). The point where the sperm aster first appears sometimes shows a considerable lateral displacement from the point of entry as marked by the exudation cone. This phenomenon was observed in the starfish by R. Chambers (1933), who found that the displacement occurred when the spermatozoon moved in with its motile tail.

The diminutive sperm aster has a small central lake (Fig. 1A). As the aster grows in size and moves farther into the egg, the lake enlarges. The rays directed on the side toward the periphery of the egg increase very much in length and extend to the periphery, while those directed toward the interior of the egg grow more slowly in length. This gives an asymmetrical shape to the aster, and the lake occupies an increasingly eccentric position toward the advancing side of the aster (Fig. 1B). As the lake approaches the centre of the egg the lake becomes again central, with the rays extending symmetrically to the periphery of the egg (Fig. 1 C). The final position of the lake approximates the centre of the egg.

Fig. 1.

Diagrammatic sketch to show growth in size of the aster and its asymmetrical intermediate stage. A. Early diminutive stage. B. More developed with eccentric lake. C. Advanced stage with central lake.

Fig. 1.

Diagrammatic sketch to show growth in size of the aster and its asymmetrical intermediate stage. A. Early diminutive stage. B. More developed with eccentric lake. C. Advanced stage with central lake.

In the spherical egg the path of the sperm aster from the point where it first became visible all the way to the approximate centre of the egg was found to be a straight line approximately at right angles to the surface.

In pronouncedly aspherical eggs the sperm aster does not always move directly to the approximate egg centre. Fig. 2 illustrates a case where a spermatozoon entered to one side of the middle of a flattened surface of the egg. The inward path taken by the enlarging sperm aster at first was at right angles to the surface and, consequently, not directed toward the approximate centre of the egg. This path is represented in the figure by the line A to B. Eventually the path becomes directed toward the centre of the egg along the path B to C. In eggs flattened by compression in the hanging-drop the sperm aster does not move into the approximate centre of the egg.

Fig. 2.

Deformed egg having one flattened surface. A, exudation cone at site of sperm entry to one side of flattened surface. AB, path of sperm aster being off the centre, later changes in direction and becomes directed toward the centre of egg along path B–C.

Fig. 2.

Deformed egg having one flattened surface. A, exudation cone at site of sperm entry to one side of flattened surface. AB, path of sperm aster being off the centre, later changes in direction and becomes directed toward the centre of egg along path B–C.

In all observed cases the speed of the sperm aster was found to be approximately uniform from the point where it first became visible all the way to the approximate egg centre.

(2) The movement of the egg nucleus

As the egg nucleus from a point of rest moves through the cytoplasm to the periphery of the aster it is accompanied by cytoplasmic granules moving in the same direction and at the same rate. As the egg nucleus moved through the aster no such synchronous movement of the granules with the egg nucleus could be observed.

The egg nucleus moves at a varying velocity from rest to its final position within the lake. The egg nucleus starts slowly from rest and moves with increasing velocity through the cytoplasm beyond and in the outskirts of the aster. The velocity decreases rapidly as the egg nucleus approaches the lake boundary, becoming nearly zero at the boundary. The final position of the egg nucleus is in the lake. An observation made on the Arbacia egg at Wood’s Hole showed that the egg nucleus while moving into the lake carried a number of agglutinated cytoplasmic granules ahead of it.

The velocity of the egg nucleus as it moves toward the aster is greater the greater the size of the aster, the size being measured at the moment the egg nucleus starts from rest.

A striking illustration of this was obtained in eggs with artificially produced exovates. The exovates were produced by compression caused by the withdrawal of water from a hanging-drop containing recently inseminated eggs. In one case, shown in Fig. 3, the egg nucleus happened to be in the exovate and the sperm aster in the main body of the egg. The egg nucleus started to migrate from the exovate toward the lake only after the aster had reached maximum development, i.e. when the astral radiations had extended to the periphery of the egg. The maximum velocity of the egg nucleus as it moved toward the aster (Fig. 3, positions 4–7), was far greater than that observed in normal eggs where the aster is never fully developed at the moment when the egg nucleus starts to move toward it. Further, the speed of the nucleus, as it approaches the lake boundary (Fig. 3, positions 7–10), is far slower than when the egg nucleus takes the same course through an earlier and less developed aster. Any change in the contour of the egg during these observations was not recorded.

Fig. 3.

Fertilized egg with egg nucleus initially in exovate. Numerals represent positions of egg nucleus, showing its advance toward lake with varying acceleration, recorded at 30 sec. intervals. Lake of sperm aster drawn in broken line.

Fig. 3.

Fertilized egg with egg nucleus initially in exovate. Numerals represent positions of egg nucleus, showing its advance toward lake with varying acceleration, recorded at 30 sec. intervals. Lake of sperm aster drawn in broken line.

Another case of an exovate was observed by Chambers (1938) in the egg of Arbacia punctulata. The sperm aster lay in an exovate while the egg nucleus was in the main body of the egg within the fertilization membrane. With the progressive enlargement of the sperm aster the exovate increased in size. The egg nucleus then rapidly moved through the constriction of the dumbbell-shaped egg and steadily advanced through the aster to the lake. Eventually all the cytoplasm within the fertilization membrane became incorporated in the exovate so that the entire egg became external to its fertilization membrane.

The resting egg nucleus is spherical. As it moves through the cytoplasm before reaching the visible radiations of the aster it exhibits irregular, slight changes in shape ; these were also observed by Selenka (1878) and by Wilson & Mathews (1895), in Toxopneustes (Lytechinus) variegatus. When the nucleus moves into the aster it becomes ovoid, and as it approaches the lake boundary it becomes pear-shaped, the apex being directed toward the lake. When the nucleus enters the lake it once more becomes spherical.

Dr R. Chambers made some observations on this point which he kindly permitted me to mention here. He injected minute droplets of paraffin oil into the fully developed sperm aster of the Lytechinus egg. Oil-drops placed among the radiations assumed an ellipsoidal shape, while those deposited in the astral lake were spherical.

(3) The movement of the egg nucleus in relation to the sperm aster

Fol (1879), Selenka (1878), and R. Chambers (1917) indicated that the egg nuclei, respectively, of Asterias, Toxopneustes and Echinarachnius does not start to move toward the sperm aster until they are reached by the astral rays. My observations on Lytechinus (Toxo-pneustes) eggs indicate that the movement of the nucleus is initiated a considerable time before the visible radiations approached the egg nucleus. In some recorded cases the egg nucleus started moving when it was about half the diameter of the egg away from the border of the visible sperm aster. Evidently the influence of the aster, at least in Lytechinus, may be exerted well beyond its visible boundaries.

No relation exists between the asymmetry of the aster and the entrance point of the egg nucleus into the aster. The egg nucleus takes a radial path through any region of the aster, irrespective of whether, in that region the radiations are well developed or are deficient.

The position of the egg nucleus at any time with respect to the sperm aster may be described by giving the angle subtended at the egg centre by the sperm aster and the egg nucleus, when the latter two are peripheral. As stated in a preliminary account (E. L. Chambers, 1937), the curvature of the path of the egg nucleus in a spherical egg depends upon this angle subtended at the egg centre. Fig. 4 shows diagrammatically a number of possible paths which the egg nucleus may take as it moves toward and into the sperm aster. When the angle subtended at the egg centre is ca. 0°or 180°, the egg nucleus moves in a straight line toward the astral lake. As the angle increases from 0° to 90° (or decreases from 360° to 270°) the curvature increases. At 90° (or 270°) the curvature is at a maximum. As the angle increases from 90° to 180° (or decreases from 270° to 180°) the curvature of the path decreases. Fig. 5 includes camera-lucida drawings of the positions of the egg nucleus and sperm aster in three eggs with the egg nucleus at different positions at the time of sperm entry. The exudation cone indicates the site where the spermatozoon entered. The pairs of numerals designate the simultaneous positions of the egg nucleus and of the sperm aster, the successive positions of the sperm aster being designated with prime numerals. Fig. 5 A shows the curved path taken by the egg nucleus during its advance from a lateral position toward the enlarging astral lake indicated by the broken circles 1′ to 7′. Fig. 5B illustrates a case in which the egg nucleus occupied an initial position almost at the opposite pole of the egg from the site of sperm entry. Fig. 5 C shows an interesting case in which the initial position of the egg nucleus was close to the centre of the egg. It shows that the egg nucleus actually migrated away from the centre of the egg until it penetrated deep into the aster. At position 6 the egg nucleus was at the lake boundary. Therefore, the positions 7–13 are the variable positions of the egg nucleus while it was being incorporated in the advancing lake.

Fig. 4.

Diagram showing four possible paths of egg nucleus (interrupted lines) and path of sperm aster (arrow heads).

Fig. 4.

Diagram showing four possible paths of egg nucleus (interrupted lines) and path of sperm aster (arrow heads).

Fig. 5.

Camera-lucida sketch of paths of egg nucleus and sperm aster in three different eggs. Exudation cone marking site of sperm entry at top of figures. Numerals designate successive positions at 30 sec. intervals of egg nucleus, prime numerals of astral lake. First record, 1 and 1′, taken 6 · 5 min. after insemination. A. Lake drawn with broken line. Final position near centre of egg. Egg nucleus and lake in contact at 7 and 7′. B. Crosses represent centres of egg nucleus and lake. Egg nucleus and lake in contact at 7 and 7′. C. Path of egg nucleus, unbroken line. Path of astral lake, arrow heads. Only one position of lake, 6′, recorded. Egg nucleus and lake in contact at 6 and 6′.

Fig. 5.

Camera-lucida sketch of paths of egg nucleus and sperm aster in three different eggs. Exudation cone marking site of sperm entry at top of figures. Numerals designate successive positions at 30 sec. intervals of egg nucleus, prime numerals of astral lake. First record, 1 and 1′, taken 6 · 5 min. after insemination. A. Lake drawn with broken line. Final position near centre of egg. Egg nucleus and lake in contact at 7 and 7′. B. Crosses represent centres of egg nucleus and lake. Egg nucleus and lake in contact at 7 and 7′. C. Path of egg nucleus, unbroken line. Path of astral lake, arrow heads. Only one position of lake, 6′, recorded. Egg nucleus and lake in contact at 6 and 6′.

In aspherical eggs the movement of the sperm aster is not in a straight line and the path of the egg nucleus is correspondingly irregular.

A case of dispermy was observed (Fig. 6) with the two growing sperm asters at different stages of development in focus with the periphery of the egg and approximately equidistant from the egg nucleus. The egg nucleus, at first, moved along the path 1 to 5 toward the less developed aster (which travelled along the path 1″ to 15″). Soon after this the egg nucleus changed in direction and moved along the path, 5 to 17, toward the more developed aster (which travelled along the path 1′ to 15′), and eventually the egg nucleus moved into its centre.

Fig. 6.

Dispermic egg. Exudation cones shown at periphery of egg. Numerals designate successive positions of egg nucleus at 30 sec. intervals, prime numerals, of earlier developed sperm aster, double prime numerals, of later sperm aster. Note that egg nucleus first moves toward later developed sperm aster, then shifts toward earlier aster. Egg considerably compressed.

Fig. 6.

Dispermic egg. Exudation cones shown at periphery of egg. Numerals designate successive positions of egg nucleus at 30 sec. intervals, prime numerals, of earlier developed sperm aster, double prime numerals, of later sperm aster. Note that egg nucleus first moves toward later developed sperm aster, then shifts toward earlier aster. Egg considerably compressed.

Under the laboratory conditions at Tortugas with a temperature of 28–30° C. the limits of variation in time from insemination until the egg nucleus reached the lake, for the Lytechinus egg, was observed to be between 7 and 12 min. This variation is correlated with the distance of the egg nucleus from the aster. The smaller the angle subtended at the egg centre by the egg nucleus and the sperm aster (in their peripheral positions) the shorter tends to be the time taken for the egg nucleus to reach the lake.

Factors tending to decrease the variability are, first, the curvature of the path of the egg nucleus as it moves toward the astral lake and second, the variation in velocity of the egg nucleus. These two factors affect the time for the egg nucleus to reach the astral lake by shortening it when the distance is great and by lengthening it when the distance is small. Therefore, there is more uniformity in the time taken for the egg nucleus to move into the lake than would be expected if the distance alone were taken into account.

(4) Analysis of the forces moving the egg nucleus from its initial position to its final position within the centre of the egg

A geometrical analysis of the paths of the egg nucleus and sperm aster is presented in Fig. 7. The positions in the figure are from a camera-lucida drawing of an actual case. As the egg nucleus travelled from positions 2–6, it was moving through the cytoplasm beyond and in the outskirts of the aster. As it travelled from positions 7–9, it was moving well within the aster. Lines are drawn between the corresponding positions of the sperm aster and the egg nucleus, i.e. from 2 to 2′, 3 to 3′, etc. The lines drawn from positions 2, 3, 4, 5, and 6 are all found to be tangents to the curve described by the path of the egg nucleus, but the lines drawn from positions 7, 8 and 9 are not tangents. For example, a tangent drawn from position 8 intersects at a distance 8′ to Y ahead of the position of the sperm aster 8′.

Fig. 7.

Camera-lucida sketch of positions of egg nucleus and sperm aster. Same as Fig. 5, but showing geometrical analysis of the two paths. Egg nucleus and lake in contact at 9 and 9′. First record, 1 and 1′, taken 6·0 min. after insemination.

Fig. 7.

Camera-lucida sketch of positions of egg nucleus and sperm aster. Same as Fig. 5, but showing geometrical analysis of the two paths. Egg nucleus and lake in contact at 9 and 9′. First record, 1 and 1′, taken 6·0 min. after insemination.

The deviation of the path of the egg nucleus directed ahead of the sperm aster is also illustrated in Fig. 5 A, B and C. In Fig. 5 A and B, this is shown as the egg nucleus moves from positions 6–7. In Fig. 5C, the deviation is shown most strikingly. The path from 4 to 6 is actually directed in the opposite direction from the lake. In the three cases, as the egg nucleus moved from positions 6–7 in Fig. 5 A and B, and from positions 4–6 in Fig. 5 C, it was well within the aster.

An added point to be emphasized in Fig. 7 is that the lines 7–7′, 8–8′ and 9–9′ are parallel to each other, whereas the lines from positions 2–2′ up to 6 and 6′ are not.

The observation was made that the slope of the parallel lines, Fig. 7, 7–7′, 8–8′ and 9–9′ is the same as the slope of the astral rays along which the egg nucleus moves (where the path of the lake is the Y axis). Evidently the egg nucleus moves radially through the aster toward the lake following the radial configuration of the aster.

Consideration of these data shows the following. During the initial stages of the movement of the egg nucleus (Fig. 7, positions 2–6) through the cytoplasm beyond and in the outskirts of the aster, a single component of force directed toward the lake acts on the egg nucleus, and this component constantly changes in direction corresponding to the movement of the aster.

During the latter stages of movement of the egg nucleus within the aster (positions 7–9) a second component of force acts on the egg nucleus in addition to the first. The parallel relation of the lines, 7–7′, 8–8′ and 9–9′ demonstrates that this second component acts on the egg nucleus carrying it centrad parallel to the path of the lake and at the same rate as the lake moves. This second component is the mass movement of the inward migrating aster. While the egg nucleus is in the aster, the first component (directed toward the lake) remains constantly unchanged in direction, since the egg nucleus and lake are both being shifted inward by the migrating aster parallel to each other and at the same rate. The path of the egg nucleus through the aster (Fig. 7, positions 7–9) is a resultant of these two components. Therefore, the curvature of the path is greater than would be expected if the only acting factor were the first component.1

Finally, when the egg nucleus comes to rest within the lake, it is moved only by the second component of force carrying it toward the approximate centre of the egg. This is shown in Fig. 5 A, as the egg nucleus moves from position 7 to the final position, and in Fig. 5C, positions 7–13.

A model may be constructed which reproduces the path described by the egg nucleus. Several rays converging toward the centre are cut out of a circular piece of heavy cardboard. This represents the aster. The point of a pencil held in the right hand is thrust through a hole in the centre of the “aster”, and the point placed in the upper right-hand corner of a fixed sheet of cardboard. Another pencil representing the egg nucleus is held in the left hand at the lower left comer of the sheet. An elastic band is stretched between the two pencils. The “aster” is now steadily moved down the right-hand margin of the sheet and the pencil at the lower left comer of the cardboard is allowed to move toward the moving “aster” pulled by the elastic band. The positions of the centre of the “aster” and of the pencil are simultaneously marked at successive intervals. As soon as the pencil has been drawn into one of the cut-out rays of the “aster” it will not only be pulled radially toward the centre of the aster, but it will also be carried down the margin of the sheet by the advancing “aster”. Analysis of the paths described by the pencil and the centre of the circular piece of cardboard, disregarding the velocity factor, gives the same relations as those found to exist between the paths of the egg nucleus and sperm aster in the Lytechinus egg.

The evidence presented by R. Chambers (1917), that the aster is a gelated body, radial in structure and progressively increasing in consistency from the periphery of the aster inward to the lake boundary, receives confirmation from the observations described in this paper.

Further, the observations indicate that the sperm or fusion nucleus, surrounded by the aster, is moved as though impelled forward by the growth in the size of a gelated body, the aster. A centrically situated body such as the aster uniformly increasing in radius within a hollow sphere must move in such a way that the centre of the growing body travels from the periphery along one of the radii of the sphere. However, when the sphere is deformed the path of the enclosed growing body is at right angles to the nearest surface only during the initial phases of development. Later, as the body continues to increase in size, its outer boundary may come into contact with surfaces in such a manner as to cause a profound alteration in the direction of its inward path. Reference to p. 412 describing the movement of the aster in spherical and aspherical eggs shows that the paths described by the growing sperm aster are similar to those of such an enlarging body.

A number of descriptions secured from the literature, namely those of Brachet (1910) on the frog’s egg, Ziegler (1895), v. Erlanger (1897), and Spek (1918) on the nematode egg, may be interpreted as lending support to the view that the movements of the sperm and fusion nuclei are due to the growth in size of a gelated body, the aster. None of these observers ascribed this significance to their observations.

Brachet depicted a frog’s egg penetrated by two spermatozoa a very short distance from each other. The inward paths, made visible by the pigment granules carried along by the sperm heads, at first ran parallel to each other. With the development of the aster about each sperm nucleus the paths diverged as if the growing asters were pushing each other apart. Spek in the course of studies on the nematode egg (Rhabditis dolichura, and others), described a retreat of the sperm nucleus from the periphery of the egg against a weak streaming of the cytoplasm toward the periphery. He stated that the granules, throughout a region extending to the periphery of the egg surrounding the moving sperm nucleus, were carried inward with the sperm nucleus. Spek concluded that this movement was due to some force other than a cytoplasmic flow. His arguments were that (1) the granules travelling with the sperm nucleus moved more slowly than granules carried along by the streaming, (2) the movement of the granules was not accompanied by a corresponding change in the contour of the egg, and (3) the movement of the granules occurred against a cytoplasmic flow. Spek’s observation is striking evidence for the existence of a region of gelation around the moving sperm nucleus. Erlanger’s description of the appearance of a double aster around the peripheral sperm nucleus in the eggs of Rhabditis dolichura and R. pellio lends additional support to this conclusion. Ziegler and v. Erlanger showed that the centering of the fusion nucleus in the nematode egg is accompanied by the growth of the amphiaster and that the long axis of the amphiaster eventually corresponded with the long axis of the egg. Indeed, both investigators described cases where the amphiaster rotated through 90 ° during growth in order to reach this position of equilibrium within the egg.

Since the movement of the aster with the enclosed sperm nucleus is due to the growth of the aster, we should not expect the sperm aster to show any deviation toward the egg nucleus in the spherical echinoderm egg. Fol (1879), in studies on Asterias glacialis, and Wilson & Mathews (1895), in studies on Toxopneustes (Lytechinus) variegatus, have described a deviation in the path of the sperm aster, but I have never observed any such deviation.

Wilson, generalizing from his own work and that of other investigators, stated in his book on the cell (1925) that the track of the sperm nucleus is typically curved and may be resolved into two components : one, a penetration path, nearly vertical to the surface of the egg not influenced by the egg nucleus, and two, a copulation path influenced by the egg nucleus, along which the sperm nucleus moved to the point of union with the egg nucleus.

It has been shown that the irregularity of the egg’s contour may produce pronounced deviations from the normal in the path of the sperm aster. This may explain Fol’s and Wilson’s observations, since these investigators did not differentiate between spherical and aspherical eggs. Irregularities in contour of the egg are far more likely to occur when the eggs are placed between slide and cover-slip than when free in hanging-drop. Fol states that he used a slide and cover-slip. Wilson does not mention this phase of his technique. Neither mentioned whether any precautions were taken to prevent compression. In this connection Selenka’s (1878) observations on the eggs of Toxopneustes (Lytechinus) variegatus are particularly significant. He made observations on the eggs in hanging-drops. He stated that the sperm aster always moves centripetally to the centre and that in some cases the aster may move all the way to the centre before the egg nucleus approaches it.

Conklin (1898) suggested that the pronuclei in the Crepidula egg were drawn together by general movements of the cytoplasm. R. Chambers (1917, 1938) presented experimental evidence suggesting that the formation and growth of the aster in the Echinoderm egg consists in the separating out, during gelation, of a hyaline liquid which flows in innumerable channels to accumulate in the astral lake.

The evidence collected in the summary of this paper regarding the movement of the egg nucleus sustains Conklin’s suggestion and R. Chambers’s conclusion.

An observation that the movement of the egg nucleus may be independent of cytoplasmic streaming was made by Wilson (1901). He stated that the moving egg nucleus may clearly be seen “to shove aside and leave behind the cytoplasmic granules” in the egg of Toxopneustes (Lytechinus) variegatus. However, he does not state whether his observations were made while the egg nucleus was moving within the aster toward the lake. If the latter were the case, Wilson’s observation is what would be expected. The granules in the aster are embedded in a gel and the hyaloplasm alone flows into the lake. The hyaloplasm carries the egg nucleus as it advances through the sperm aster past the embedded granules. My observation that the granules move with the egg nucleus was made only while the egg nucleus moved through the cytoplasm beyond the confines of the astral boundary.

It is peculiar that the egg nucleus is able to move in the aster, when the far smaller granules do not. R. Chambers (1917) observed that if one of the small oil-like droplets, occasionally found in the cytoplasm of the Echinarachnius egg, is pushed by the needle from the liquid cytoplasm into the periphery of the aster the droplet will move toward the lake. He did not observe its passage into the lake. Further investigation is required to determine why the egg nucleus is the only body within the egg which is carried all the way through the aster into the astral lake.

A remarkable observation was made by Boveri (1918) on the eggs of Parechinus microtuberculatus which fits in admirably with the explanation offered in this paper to account for the movement of the egg nucleus. Boveri found a female with eggs containing several partial nuclei variously distributed within the egg. He fertilized these eggs and found that as the sperm aster moved to the egg centre the partial nuclei migrated into the astral lake, where they all united with the sperm nucleus to form a single fusion nucleus. This movement of the partial nuclei from various regions in the egg toward and into the astral lake is well explained by the existence of a streaming of the cytoplasm into the aster.

Wilson (1901) described cases where motion of the egg nucleus may occur in the absence of an aster. He suppressed the development of the sperm aster by etherizing eggs of Toxopneustes (Lytechinus) variegatus 1 min. after fertilization. He observed that if the spermatozoon entered close to the egg nucleus, it moved toward and fused with the sperm nucleus, but did so only after a lapse of hrs. He also stated that no motion was observed when the egg nucleus was at a considerable distance from the point of entry of the spermatozoon. In these eggs there is no indication of a complete cessation of the cytoplasmic flow. The streaming, carrying the egg nucleus toward the sperm nucleus, may be so weak that the granules in the eggs are not sufficiently aligned to show the sperm aster.

Moore (1937) made the following interesting observations on eggs inseminated after they had been deformed by centrifuging to a dumbbell shape. The lighter portion of the egg contained the egg nucleus and was joined to the heavier and larger portion by a constricted stalk of cytoplasm. In one case the sperm entered at the lighter end and fusion of the sperm and egg nuclei occurred there. The fusion nucleus then moved through the connecting stalk into the larger portion of the egg. Moore speaks of a “pull on the fusion nucleus sufficient to bring it across” the stalk. However, the movement of the fusion nucleus may be explained by the growth of the aster about it. This would require an enlargement of the stalk, if too narrow, to allow the growing aster to slip through. In a second case recorded by Moore the sperm entered at the heavy pole while the egg nucleus was at the light pole. Moore found that as long as the distance between the pronuclei was 5° % greater than the diameter of the egg, they did not come together. A glance at his sketches, however, shows that the stalk connecting the two poles of the egg is so narrow, that movement of the nuclei through this stalk could hardly be expected. More investigation is required to ascertain the relation of the movement of the egg nucleus to cytoplasmic streaming in parthenogenetically activated eggs, and the study of the migration of the pronuclei in flask-shaped eggs constitutes an important problem for further research.

The discussion on the migration of the egg nucleus to the sperm nucleus may be brought to an end by drawing attention to the observations of Auerbach (1874), Ziegler (1895), v. Erlanger (1897), and Spek (1918) which clearly show that the movements of the egg nucleus in the nematode egg depend upon cytoplasmic streaming. At a certain stage in the development of the peripherally lying sperm nucleus, a sustained, fountain-like streaming of the cytoplasm develops flowing axially toward and peripherally away from the sperm nucleus. The axial flow carries the egg nucleus to the sperm nucleus whereupon fusion occurs.

In the echinoderm egg no countercurrent (peripherally away from the lake) has been observed to compensate for the centripetal flow which carries the egg nucleus to the astral lake. The only phenomenon accompanying the centripetal flow is the enlargement of the astral lake to include the inwardly flowing hyaloplasm.

  1. The following observations indicate that the egg nucleus is moved into the astral lake by a centripetal cytoplasmic flow directed toward the lake.

    • A. The cytoplasmic granules move with the egg nucleus in its vicinity as the egg nucleus migrates from rest to the periphery of the aster.

    • B. The egg nucleus starts to move toward the sperm aster long before it is reached by any visible radiations, and it moves with increasing velocity as it travels from rest through the cytoplasm into the outer region of the aster.

    • C. The egg nucleus always moves toward the lake. Its curved path, and the dependence of the curvature on the relative initial positions of the egg nucleus and sperm aster result from the movement of the egg nucleus toward a lake which itself is moving. In deformed eggs the path of the aster may be deflected from a straight line with a corresponding irregularity in the path of the egg nucleus. In dispermic eggs the egg nucleus may first move toward the lake of one aster, and later move toward and into the lake of the other aster.

  2. The,following observations indicate that the aster is a gelated body, radial in structure.

    • A. The egg nucleus when deep within the aster undergoes ovoid and pearshaped deformations, and a pronounced deceleration in its movement as the lake boundary is approached.

    • B. When the egg nucleus is within the aster, its path is influenced by the migration of the aster as a whole, as if the egg nucleus were being carried by a rigid body during the continued advance of the egg nucleus toward the astral lake.

    • C. The egg nucleus always takes a radial path through any region of the aster, following the radial configuration of the aster, as if constrained from moving in any other direction.

    • D. The path described by the egg nucleus may be duplicated in a model where the sperm aster is represented as a rigid body with radial spacings to permit entry of the egg nucleus.

  3. The following observations indicate that the lake, within which the sperm nucleus lies, is carried to the centre of the egg by the growth in size of a gelated body, the surrounding aster.

    • A. The inward migration of the lake at uniform speed is accompanied by the uniform growth of the aster, the rays extending to the nearest periphery of the egg growing faster than those on the side of the aster toward the interior of the egg. The lake is brought to its final position when the astral rays all around the lake reach the periphery of the egg.

    • B. The paths described by the sperm aster in spherical and deformed eggs are similar to those described by an enlarging rigid body within non-distorted and distorted spheres respectively.

    • C. Not only the lake, but also the aster surrounding the lake, moves inward from the periphery of the egg, since the egg nucleus while within the aster and still at a considerable distance from the lake is carried inward from the periphery of the egg parallel to the path of the aster at the same rate as the aster moves.

  4. The time required for the pronuclei to come into contact depends upon the distance between the initial positions of the two bodies, the curvature of the path of the egg nucleus, and the variation in its speed.

The observations described in this paper indicate that the sperm nucleus is carried to the centre of the egg by the progressive enlargement of a gelated body, the sperm aster, and that the egg nucleus is carried to the sperm nucleus by a centripetal cytoplasmic flow directed toward the astral lake, within which the sperm nucleus lies. The curved path which the egg nucleus describes results from the continued change in direction of a cytoplasmic flow streaming toward the moving astral lake.

The egg nucleus, when within the aster, is not only carried towards the lake by cytoplasmic streaming, but it is also moved centrally by the migration of the sperm aster toward the approximate centre of the egg.

I wish to express my thanks to my father, Dr R. Chambers, for his highly valuable suggestions during these investigations and the writing of this paper.

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1

During the summer of 1937 I had the opportunity of working at the Marine Laboratory of the Carnegie Institution of Washington on Dry Tortugas, Florida.

2

The “astral lake” or “lake” is the term used by Chambers (1917) for the hyaline region within the aster from which innumerable rays radiate. Its site is the “centrosphere” of cytologists.

1

The deceleration in the movement of the egg nucleus as it penetrates deep into the aster causes, in itself, a pronounced increase in the curvature of the path of the egg nucleus. However, if no force entered other than that carrying the egg nucleus toward the lake, the path of the egg nucleus would always be directed toward the lake.