1. Three main types of yolk drop are present in the area pellucida of the chick blastoderm. They have been given the names ‘complex yolk drop’, ‘type A yolk drop’, and ‘type B yolk drop’. Together they have been called secondary yolk to distinguish them from the extra-embryonic or primary yolk.

  2. It is concluded that the complex yolk drops are early stages in the conversion of yolk. They contain granular material which resembles that found in the primary yolk. Type A drops are present in the complex yolk drops and appear to be formed from the granular material. Type B drops, mitochondria, vacuoles, and membranes are also found within the complex drops.

  3. The membranes around the complex drops appear to break down, releasing the contents of the yolk drops into the cell.

  4. Most type A drops appear to become converted into a number of circular, or possibly tubular, bodies. These are believed to be predominantly lipoproteins. It is concluded that the membrane around each type A drop breaks down and these circular bodies are then released into the cytoplasm where they may contribute to the endoplasmic reticulum.

  5. Other type A drops are composed of numbers of micro-particles. They closely resemble the cytoplasmic micro-particles and may become part of them. Evidence is presented for considering these micro-particles to be precursors of ribonucleoproteins.

  6. The type B drops persist in the cytoplasm after the complex drops and the type A drops have almost completely disappeared. Evidence is presented for considering them to be fatty drops.

  7. Primary yolk from the hen’s egg has also been examined.

In almost all embryos yolk becomes converted into cytoplasm. It has not previously been possible to describe in any detail the morphological changes involved in this process; indeed, when the yolk drops contained within embryonic cells are examined by light microscopy they seem to remain in much the same condition until they are suddenly used up. For this reason they have frequently been considered to be nothing but ‘inert, inactive’ stores of food. By using an electron microscope, however, it has been possible to trace some of the morphological changes which take place in the chick when intra-cellular yolk drops are converted into cytoplasm, and to show that these are not confined to a single stage of embryonic development. Moreover, the discovery of mitochondria within the yolk drops suggests that the yolk drops are not ‘inert’.

The following stages have been examined: medium and long primitive streak (as defined by Waddington, 1932, and Abercrombie, 1950), head process, head fold, and 10-16 pairs of somites. Observations have been mainly confined to the primitive streak or to the axial tissues of the embryos, although the area opaca has also been examined. Forty-eight specimens have been used. White yolk and yellow yolk from the yolk sac of both unincubated and incubated eggs has also been studied.

Most specimens were fixed in osmic acid buffered with sodium veronal (Palade, 1952) to a pH of 7·4 and subsequently embedded in methacrylate. A few embryos were, however, fixed in potassium permanganate (Luft, 1956) at a pH of 7·4 and embedded in the epoxy resin, ‘Araldite’ (Glauert, Rogers, & Glauert, 1956). Ultra-thin sections were examined in a Siemens Elmiskop lb electron microscope. Unfixed yolk and blastoderms were also studied by light microscopy.

The yolk which lies beneath the blastoderm will be called primary yolk. The yolk drops in the blastoderm, which are usually intracellular, will be known as secondary yolk.

The primary yolk

Plate 1, fig. A is an electron micrograph of primary yolk from an unincubated egg. This material was examined as a smear before fixation, and was seen to consist of a liquid in which two kinds of yolk drop were floating. These three components can be recognized in the electron micrograph. The liquid seen by light microscopy corresponds with the continuous granular material {con. ph.). One set of yolk drops as seen by light microscopy were round, glistening, and compact; these are considered to be fatty drops (see Discussion) and are labelled f in the electron micrograph. The second kind were also spherical but considerably larger and translucent; within each of them were several fatty drops. I call these ‘primary yolk drops’ as opposed to the secondary yolk drops found within the embryonic cells (see below). In the electron micrograph it can be seen that in addition to the enclosed fatty drops the primary yolk drops contain a granular material {gr.) similar to the surrounding fluid {con. ph.) although less dense. No membranes could be seen around the primary yolk drops. These yolk drops were more common in non-incubated yellow yolk than in white yolk, or than in yellow yolk which had been incubated for 24 hours. Yolk drops of this type were never seen inside embryonic cells.

The secondary yolk

The yolk drops found within the cells of the area pellucida can be divided into three main types, the first two of which can be seen in Plate 1, fig. B, which is a section across the primitive streak of a chick blastoderm in the long primitive streak stage.

  1. Type A has either a dense core surrounded by a less dense region, or a granular appearance which is more or less uniform throughout. These yolk drops tend to be oval in shape after methacrylate embedding, but by shifting the block face through 90° this was found to be due to compression during sectioning. After embedding in Araldite the yolk drops were seldom compressed.

  2. Type B usually has a very electron dense appearance after osmic fixation and is homogeneous throughout. The irregular outline of the type B drops is probably an artefact (see Discussion). These drops may be derived from the fatty drops in the primary yolk.

Occasionally type A drops are seen to contain type B drops within them (Plate 5, fig. M). In most sections the type A drops show a wider variation in size than the type B drops and are generally larger; in Plate 1, fig. B, for example, the long axis of the type A drops varies from to 3 μ, that of the type B drops from to 1 μ. The largest type B drops seen in the area pellucida, however, had a maximum diameter of 5 μ, but this is unusual. The average size of the yolk drops is smaller at the hind end of the primitive streak than farther forward.

(3) Complex yolk drops. The third type of secondary (i.e. intra-cellular) yolk drops will be called complex yolk drops, for they contain both type A and type B drops within them, as well as mitochondria, vacuoles, and various particles (Plate 2, figs. D-F). Complex yolk drops could not be recognized in the cells of living embryos examined by light microscopy, but structures closely resembling the primary yolk drops of the extra-cellular yolk could be seen.

These three types of secondary yolk drops seen in the cells of the area pellucida are similar to structures in the area opaca, although in the latter region they are larger and more plentiful. Complex yolk drops have also been seen lying between the cells of the primitive streak. Preliminary investigations show that in the area pellucida the type B drops are usually the most common, and they are still present when most of the complex and type A drops have disappeared (Table 1). In only one case were complex drops found later than the head-process stage; and in only two specimens were type A drops seen later than the head-fold stage. The type B drops also appeared to become reduced in numbers, however; for example, in the head-process stage a montage of the presumptive somite mesoderm consisting of 36 cells (the section passing through the nucleus in 11 of these) contained 103 type B drops. In a montage of a somite at the 12 pairs of somites stage, however, consisting of 295 cells (105 of these cut through the nucleus) only 15 type B drops could be found.

Table 1.

The ratio of the three main types of yolk drop at various stages

The ratio of the three main types of yolk drop at various stages
The ratio of the three main types of yolk drop at various stages

The conversion of secondary yolk into cytoplasm

The secondary yolk drops can be arranged in a series which suggests that they become converted into cytoplasm.

(i) Complex yolk drops

Within each complex yolk drop can frequently be seen a region containing granular material (gr., Plate 2, fig. E) which closely resembles that in the primary yolk drops (Plate 1, fig. A). Both type A and type B yolk drops are usually present. The type A drops can be arranged in a series according to their structure (A1-A5 in Plate 2, fig. E). At one end of the series is a highly granulated kind (Al) which resembles the granular material (gr.), and at the other end is a typical type A drop (A5) indistinguishable from the many which lie freely in the cytoplasm.

Other parts of the complex drops are separated from one another by internal membranes (i.m., Plate 2, fig. E). Mitochondria can usually be seen inside the complex drops. Each complex drop is surrounded by a membrane. Membranous filaments are sometimes visible under the surface membrane (m.f., Plate 2, fig. E).

There is evidence, set out in the Discussion, for believing that the membrane around each complex yolk drop subsequently breaks down, so that the contents of the yolk drop are released into the cytoplasm.

(ii) Type A yolk drops

After the type A yolk drops have formed in the complex yolk drops, see above) various changes take place. These are continued after the release of the type A drops into the cytoplasm. Many free type A drops have a dense core and a less dense outer region (Plate 1, fig. C). The series shown in Plate 3, fig. H(1-5) suggests that there is a gradual change in the structure of the core until it resembles that of the outer part of the yolk drop. The absence of a core in the 5th stage has been confirmed by examining serial sections. Eventually, the whole yolk drop when seen in transverse section usually appears to consist of a set of circular, semi-circular, or oval bodies, which will be termed circular bodies.

Where the circular bodies touch, they give the appearance of a latticework, but frequently they exist as discreet bodies not in contact with one another. Occasionally they appear to overlap. The circular bodies are not always the same size in adjacent yolk drops, but are usually much the same in any particular yolk drop (Plate 3, fig. I). At higher magnifications the wall of each circular body appears to be made of a membrane consisting of two dark lines separated from one another by a light line, the total thickness of the membrane being about 50 to 100 A, (Plate 4, fig. L).

Some type A drops contain membranes of varying lengths (Plate 4, fig. K). Each of these membranes is about the same thickness as the wall of the circular bodies. It is possible therefore that some at least of the circular bodies are really tubules seen in transverse section. In some yolk drops both membranes and circular bodies are visible in the same section (Plate 4, fig. L).

The type A drops are usually surrounded by a membrane (Plate 3, fig. J and Plate 4, fig. L) which has the same structure as the walls of the circular bodies. In some cases where the yolk drop has become completely converted into circular bodies the outer membrane appears to have broken down (Plate 3, fig. J). The contents of the yolk drop have therefore just been liberated into the cytoplasm. They closely resemble the endoplasmic reticulum.

Not all the type A drops become converted into circular or tubular structures, however. In some cases yolk drops are found which contain many tiny particles lying separately from one another. These will be known as micro-particles’, they are about 100 to 150 A in diameter. In these type A yolk drops type B yolk drops are frequently found (Plate 5, fig. M); the surrounding membranes are usually double, each part consisting of two dark lines separated by one light line.

These micro-particles in the yolk drops closely resemble similar structures in the cytoplasm, which will be called cytoplasmic micro-particles. After the type A drops have disappeared the number of these cytoplasmic micro-particles rises, in some regions at least. For instance, Plate 5, fig. O, shows part of the presumptive neural-plate ectoderm in the primitive streak stage (see Text-fig. 1 A, B). Plate 5, fig. P is a section across the hind-brain of an embryo at the stage of eleven pairs of somites (see Text-fig. 1 c, D). In the later stage the type A drops have completely disappeared and the number of cytoplasmic micro-particles has increased.

Text-fig. 1.

A, diagram of area pellucida at the full length primitive streak stage, B, diagram of a transverse section across the primitive node. The rectangle shows the position in the embryo of the sections illustrated in Plate 1, fig. B and Plate 5, fig. O. C, diagram of an embryo with 11 pairs of somites, D, diagram of a section across the hind-brain of the embryo shown in c at the level indicated by the arrow. The rectangle demonstrates the position in the embryo of the section illustrated in Plate 5, fig. P.

Text-fig. 1.

A, diagram of area pellucida at the full length primitive streak stage, B, diagram of a transverse section across the primitive node. The rectangle shows the position in the embryo of the sections illustrated in Plate 1, fig. B and Plate 5, fig. O. C, diagram of an embryo with 11 pairs of somites, D, diagram of a section across the hind-brain of the embryo shown in c at the level indicated by the arrow. The rectangle demonstrates the position in the embryo of the section illustrated in Plate 5, fig. P.

In Plate 2, fig. G, a mitochondrion-like body lies between the two layers of the outer membrane. Mitochondria were found within all varieties of type A drops.

(iii) Type B yolk drops

The type B drops have been examined mainly after osmic acid fixation and methacrylate embedding. Internally the type B yolk drops consist of a regularly arranged and apparently continuous substance. Heavily osmiophilic granules are also present, especially around the edge (Plate 5, fig. N). After fixation in permanganate, membranes were visible around the type B drops although this reagent failed to fix the ‘continuous substance’ (Plate 3, fig. J).

When they lie freely in the cytoplasm type B drops possess a more irregular outline than when they are forming part of a complex yolk drop. In serial sections taken through a single type B drop it was found that the irregularity extended over the whole surface. To make any accurate assessment therefore of the changes in size of the type B drops during the development of the embryo, it would be necessary to carry out a series of reconstructions at various stages. Nevertheless, I gained the impression that the type B drops became smaller as the embryo grew older. Evidence will be presented in a subsequent paper that they also become reduced in numbers.

The conclusions which will be reached are summarized in Text-fig. 2.

Text-fig. 2.
Diagram to summarize the way in which yolk appears to be converted into cytoplasm in the cells of the area pellucida. The transformations proposed are as follows:
  1. Primary yolk drop f., fatty drop; gr., granular material.

  2. Complex yolk drop within a cell, o.m., outer membrane of yolk drop; n., cell nucleus. The type B drops are probably derived from the fatty drops of the primary yolk drop. The type A drops (Al, A2, A3, A4) appear to develop from the granular material (gr.). Mitochondria (m.), membranous filaments (m.f.), and small vesicles (v.) are also present. The same cell after the contents of the complex yolk drop have entered the cytoplasm.

  3. Many type A drops (A5-A7) become converted into circular bodies (c.b.). These may later become endoplasmic reticulum. Others G4) consist of yolk micro-particles (y.m.) which subsequently may become cytoplasmic micro-particles (c.m.).

  4. Higher magnification of circular bodies to show the structure of their walls.

Text-fig. 2.
Diagram to summarize the way in which yolk appears to be converted into cytoplasm in the cells of the area pellucida. The transformations proposed are as follows:
  1. Primary yolk drop f., fatty drop; gr., granular material.

  2. Complex yolk drop within a cell, o.m., outer membrane of yolk drop; n., cell nucleus. The type B drops are probably derived from the fatty drops of the primary yolk drop. The type A drops (Al, A2, A3, A4) appear to develop from the granular material (gr.). Mitochondria (m.), membranous filaments (m.f.), and small vesicles (v.) are also present. The same cell after the contents of the complex yolk drop have entered the cytoplasm.

  3. Many type A drops (A5-A7) become converted into circular bodies (c.b.). These may later become endoplasmic reticulum. Others G4) consist of yolk micro-particles (y.m.) which subsequently may become cytoplasmic micro-particles (c.m.).

  4. Higher magnification of circular bodies to show the structure of their walls.

The identity of the secondary yolk drops

The secondary (or embryonic) yolk drops described in this paper were found in the area pellucida; they are considered to be partially converted yolk, for two reasons. First, these structures closely resemble similar ones in the area opaca which is a region containing so many yolk drops that it is tinged yellow by them when alive; the resemblance is apparent when the tissues are examined both by light and electron microscopy. Secondly, some of the secondary yolk drops contain a granular material which is similar to the granular material in the primary yolk drops (cf. gr. in Plate 1, fig. A and Plate 2, fig. E).

Yolk is present in the blastodisc cells from the beginning of cleavage (Kono-packa, 1933). It is generally accepted that the cells of the area opaca take up the primary yolk by phagocytosis (Remotti, 1927; Schechtman, 1956), but there is little information as to whether the area pellucida cells do so too.

Complex yolk drops are believed to be derived from the primary yolk drops (see below). Complex yolk drops could not be recognized in cells studied by light microscopy, and have not been described by previous workers; they are indistinguishable from primary yolk drops under these conditions. From the present study, however, it can be seen that yolk drops with the same structure as the primary yolk drops are not present inside the embryonic cells, at least at the stages examined.

The conversion of the secondary yolk drops into cytoplasm

It has been shown that the number of yolk drops in the area pellucida gradually falls, until by the stage of 10-16 pairs of somites they have practically disappeared. This gradual reduction in the numbers of yolk drops has been reported previously by Konopacka (1933), Knorre (1951), and Lavarack (1957).

On the evidence listed below the complex drops are believed to be derived from the primary yolk drops and to give rise to some at least of the type A and type B drops. An alternative hypothesis would be that the complex drops are the result of a collecting together of the type A and type B drops, together with other cellular components such as mitochondria. There are four lines of evidence.

  1. Each complex drop contains granular material similar to that in the primary yolk (cf. Plate 1, fig. A and Plate 2, fig. E).

  2. Each complex drop is surrounded by a membrane; if the second hypothesis were accepted it would be necessary to postulate that a membrane arose and encircled a group of type A and type B yolk drops within the cell. If the first hypothesis is correct, however, a more likely proposition can be put forward, namely, that the membrane arose at the interface between the yolk drop and the surrounding cytoplasm.

  3. Complex drops, though plentiful in the primitive-streak and head-process stages, were found only once in later stages; type A and type B drops were, however, still present. This would be expected if the complex drops were being broken down into type A and type B drops.

  4. The breakdown of yolk drops has been reported by Celener (1945), and by Grodzinski (1947) who grew yolk sac endoderm cells in tissue culture. Grodzinski reported that the membrane around the yolk drop broke down and the contents were liberated into the cytoplasm.

The chemical nature of the yolk drops

According to Grodzinski (1947) the primary yolk contains fatty drops which may either float freely in a liquid protein or lie within larger yolk drops. It is concluded, therefore, that the heavily osmiophilic bodies shown in Plate 1, fig. A, are these fatty drops.

Fatty drops are also known to occur in the cytoplasm of the area pellucida (Fraser, 1956) and within some of the secondary (intra-cellular) yolk drops (Konopacka, 1933; Thomas, 1938). They have been reported to persist after the rest of the secondary yolk has disappeared (Konopacka, 1933). The type B drops fulfil these conditions and are therefore judged to be predominantly fatty. Moreover, they resemble similar structures seen in electron micrographs of tissues known to have a high fat content (Lever, 1957). The appearance of the fatty drops in the primary yolk is, however, not identical with that of the type B drops. It is concluded therefore that, although both are fatty, they do not have exactly the same structure.

Apart from the fatty drops and a small amount of carbohydrates, primary yolk consists largely of proteins or lipo-proteins (Needham, 1931, 1950; Kono-packa, 1933; Thomas, 1938; Grodzinski, 1947). The type A drops appear to be derived from the fluid part of the primary yolk drops (gr., Plate 1, fig. A) and are therefore considered to be predominantly protein or lipo-protein mixtures.

The conversion of the type A drops into cytoplasmic structures

The present investigation shows that the formation of the type A drops may take place within the complex drops. This appears to be the process described by Konopacka (1931) for the yolk drops of the area opaca cells. She found that the protein material formed into large granules, after which the whole yolk drop broke down and the granules became vacuolated and disappeared.

(i) The circular bodies

The transformation of the type A drops into the circular bodies may in some cases start inside the complex drops, but in others take place entirely in the cytoplasm after the complex drops have broken down. It is also possible that some of the type A drops lying freely in the cytoplasm are derived directly from the ingestion of the proteins of the primary yolk rather than from the dispersal of the contents of the complex yolk drops; in such cases, however, the membrane around the type A drops must have arisen in a different way.

The walls of each circular body appear to consist of a membrane composed of two dark lines separated by a light line. Membranes of this type were first described by Robertson (1957,1958) in nerve-fibres. Because they contain membranes it is considered that the type A drops consist predominantly of lipoproteins.

The circular bodies appear to be released into the cytoplasm. They closely resemble the endoplasmic reticulum; it is possible, therefore, that they contribute to this system although there is no evidence of any rise in the amount of endoplasmic reticulum just after the yolk drops have disappeared.

(ii) The micro-particles

Some of the type A drops appear to be transformed into micro-particles rather than into circular bodies. The genesis of these yolk drops has not been followed, but it is possible that they arise from those parts of the complex drops whose fate has not been traced, for instance, the dense material (d) seen in Plate 2, fig. D. The micro-particles in the yolk drops are of comparable size and appearance to the cytoplasmic micro-particles. After the type A drops have disappeared there is a striking increase in the numbers of cytoplasmic microparticles, especially in the neural tube. It is suggested, therefore, that the yolk micro-particles enter the cytoplasm and become converted into cytoplasmic micro-particles.

The cytoplasmic micro-particles are believed to be ribonucleoproteins for two reasons. First, there is evidence that similar structures found in other tissues are ribonucleoproteins (Palade, 1955; Palade & Siekevitz, 1956). Secondly, there is a rise in the amount of ribonucleoproteins in the neural tube (Gallera & Oprecht, 1948; Brachet, 1950; Lavarack, 1957) at the time when the numbers of cytoplasmic micro-particles are increasing. According to Lavarack this rise in ribonucleoproteins occurs as the yolk drops break down. It is suggested, therefore, that yolk micro-particles become converted into ribonucleoproteins at the time when they are released into the cytoplasm. It is relevant to note here that there is some biochemical evidence that the yolk of the hen’s egg, as well as that of other vertebrates, may become converted into ribonucleoproteins (Brachet, 1950).

The fate of the type B yolk drops

According to Grodzinski (1947) lipase breaks down the surface of the fatty drops in the primary yolk and causes it to fuse into larger bodies. It seems unlikely that the type B drops in the area pellucida cells fuse together in this way, however, for as the embryos became older, the type B drops appeared to become smaller. They also appeared to become fewer in number, so that it is possible that their substance gradually seeps away into the cytoplasm. Here perhaps it remains as small isolated granules, or it is possible that it becomes attached to other structures in the cytoplasm.

Grodzinski (1947) concluded that fatty drops become transformed from glycerides to phosphatides. Fraser (1956) has suggested that cytochrome oxidase is attached to the fatty drops in the young chick blastoderm, but according to Spratt (1952) this enzyme may be associated with the mitochondria in the chick.

The origin of the mitochondria found within the yolk drops

Mitochondria have not previously been reported to be present inside yolk drops. The present work shows that they may occur within the complex drops and within the type A drops. There appear to be four possible explanations:

  1. The mitochondria are maternal ones. Brambell (1925), who studied oogenesis in the fowl, concluded that much of the yolk is formed directly from mitochondria. The mitochondria in the intracellular yolk drops might therefore have been present in the yolk drops from the time of oogenesis.

  2. The mitochondria have formed inside the yolk drops. This might take place by the conversion of the circular bodies.

  3. The mitochondria have formed by an invagination and subsequent proliferation of the membranes surrounding the yolk drops.

  4. The mitochondria have migrated into the yolk drops from the surrounding cytoplasm.

It may be noted here that occasionally in yolk drops which are surrounded by double membranes, small sac-like objects can be seen lying between the two membranes (Plate 2, fig. G). These may be mitochondria either forming from the membranes as in hypothesis 3, or migrating in from the cytoplasm as in hypothesis 4. The problem will be discussed more fully in a subsequent paper.

It is possible that the mitochondria perform an important function in the digestion of yolk, for they are known to contain large numbers of enzymes (Gustafson, 1954).

It is a pleasure to thank Dr. J. David Robertson for his patience in instructing me in the various aspects of electron microscopy. I am also greatly indebted to him, to Professor J. Z. Young, F.R.S. (in whose department the work was carried out), and to Dr. A. J. Marshall for the critical interest which they have taken in this work. I am also most grateful to Miss Rose Smith and Mr. J. Pettitt for technical assistance, and to Miss E. R. Turlington for Text-fig. 2.

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.
Plate 1

Fig. A. Yolk from an unincubated hen’s egg showing on the left the edge of a primary yolk drop and on the right the continuous liquid phase of the yolk (con. ph.). Fatty drops (f) are found both within the primary yolk drops and lying freely in the liquid. The primary yolk drops also contain a granular material (gr.). Osmic and methacrylate. Magnification: × 11,000.

Fig. B. Transverse section through the primitive node of a long primitive streak stage blastoderm (see Text-fig. 1 A, B). The dorsal side is to the left. A, type A drop; B, type B drop; n, nucleus. Osmic and methacrylate. Magnification: × 3,600.

Fig. C. Type A yolk drop showing dense core and less dense outer region, c, core; o.r., outer region; o.m., outer membrane. Osmic and methacrylate. Magnification: × 26,000.

Plate 1

Fig. A. Yolk from an unincubated hen’s egg showing on the left the edge of a primary yolk drop and on the right the continuous liquid phase of the yolk (con. ph.). Fatty drops (f) are found both within the primary yolk drops and lying freely in the liquid. The primary yolk drops also contain a granular material (gr.). Osmic and methacrylate. Magnification: × 11,000.

Fig. B. Transverse section through the primitive node of a long primitive streak stage blastoderm (see Text-fig. 1 A, B). The dorsal side is to the left. A, type A drop; B, type B drop; n, nucleus. Osmic and methacrylate. Magnification: × 3,600.

Fig. C. Type A yolk drop showing dense core and less dense outer region, c, core; o.r., outer region; o.m., outer membrane. Osmic and methacrylate. Magnification: × 26,000.

Plate 2

Figs. D and E. Two complex yolk drops, o.m., outer membrane; i.m., internal membranes; gr., granular material resembling that of the primary yolk drops; Al, A2, A3, A4, and A5, apparent stages in the formation of type A drops from the granular material; B, type B drop; m., mitochondrion; m.f., membranous filaments; d., dense material. Osmic and methacrylate. Magnification:/), × 11,000; E, × 8,000.

Fig. F. Enlargement of the rectangle outlined in fig. D to show the mitochondria. Osmic and methacrylate. Magnification: × 36,000.

Fig. G. Type A yolk drop. At one side a mitochondria-like body can be seen between the two layers of the surrounding membrane. Osmic and methacrylate. Magnification: × 14,000.

Plate 2

Figs. D and E. Two complex yolk drops, o.m., outer membrane; i.m., internal membranes; gr., granular material resembling that of the primary yolk drops; Al, A2, A3, A4, and A5, apparent stages in the formation of type A drops from the granular material; B, type B drop; m., mitochondrion; m.f., membranous filaments; d., dense material. Osmic and methacrylate. Magnification:/), × 11,000; E, × 8,000.

Fig. F. Enlargement of the rectangle outlined in fig. D to show the mitochondria. Osmic and methacrylate. Magnification: × 36,000.

Fig. G. Type A yolk drop. At one side a mitochondria-like body can be seen between the two layers of the surrounding membrane. Osmic and methacrylate. Magnification: × 14,000.

Plate 3

Fig. H. Series of type A yolk drops showing their conversion into a group of circular bodies (A5). Osmic and methacrylate. Magnification: × 9,000. The differences in size depend mainly on whether the section passes through the middle of the yolk drop or not.

Fig. I. Three type A yolk drops each with different-sized circular bodies (c.b.) and each surrounded by a membrane (o.m.). e.r., endoplasmic reticulum. Osmic and methacrylate. Magnification: × 30,000.

Fig. J. Type A yolk drops. In 1 an outer membrane (o.m.) is present. In 11 this membrane appears to have been broken down, e.r., endoplasmic reticulum. Permanganate and Araldite. Magnification: × 36,000. Three type B yolk drops are present in the upper half of the left side of the figure.

Plate 3

Fig. H. Series of type A yolk drops showing their conversion into a group of circular bodies (A5). Osmic and methacrylate. Magnification: × 9,000. The differences in size depend mainly on whether the section passes through the middle of the yolk drop or not.

Fig. I. Three type A yolk drops each with different-sized circular bodies (c.b.) and each surrounded by a membrane (o.m.). e.r., endoplasmic reticulum. Osmic and methacrylate. Magnification: × 30,000.

Fig. J. Type A yolk drops. In 1 an outer membrane (o.m.) is present. In 11 this membrane appears to have been broken down, e.r., endoplasmic reticulum. Permanganate and Araldite. Magnification: × 36,000. Three type B yolk drops are present in the upper half of the left side of the figure.

Plate 4

Fig. K. Edge of a type A drop containing internal membranes (i.m.). Osmic and methalcrylate. Magnification: × 54,000.

Fig. L. Edge of type A drop containing membranes (i.m.) and surrounded by an outer membrane. (o.m.). The regions between the paired arrows indicate the width of each membrane, which consists of two dark lines separated by a light line. Permanganate and Araldite. Magnification: × 130,000. Inset: circular body (c.b.) from same specimen. Magnification: × 176,000.

Plate 4

Fig. K. Edge of a type A drop containing internal membranes (i.m.). Osmic and methalcrylate. Magnification: × 54,000.

Fig. L. Edge of type A drop containing membranes (i.m.) and surrounded by an outer membrane. (o.m.). The regions between the paired arrows indicate the width of each membrane, which consists of two dark lines separated by a light line. Permanganate and Araldite. Magnification: × 130,000. Inset: circular body (c.b.) from same specimen. Magnification: × 176,000.

Plate 5

Fig. M. Two type A drops, one with an enclosed type B drop, the other with an enclosed mitochondrion. Both yolk drops are surrounded by two membranes and contain within themselves micro-particles which closely resemble the cytoplasmic micro-particles. B, type B yolk drop; o.m., outer membranes; m., mitochondrion; y.m.p., yolk micro-particles; c.m.p., cytoplasmic micro-particles. Osmic and methacrylate. Magnification: × 28,000.

Fig. N. Type B yolk drop. The heavily osmiophilic granules are indicated with arrows. Osmic and methacrylate. Magnification: × 74,000.

Fig. O. Presumptive neural-plate ectoderm of a primitive streak stage blastoderm (see Textfig. 1A). Yolk drops are present and cytoplasmic micro-particles are sparse. A, type A yolk drop; B, type B yolk drop; c.m., cell membrane; n., nucleus; m., mitochondrion; c.m.p., cytoplasmic micro-particles. Osmic and methacrylate. Magnification: × 11,000.

Fig. P. Section through the hind-brain of an embryo at the stage of 11 pairs of somites (see Text-fig. 1 c, D). Yolk drops have disappeared and the number of cytoplasmic micro-particles has increased, n., nucleus; c.m., cell membrane; c.m.p., cytoplasmic micro-particles. Osmic and methacrylate. Magnification: × 11,000.

Plate 5

Fig. M. Two type A drops, one with an enclosed type B drop, the other with an enclosed mitochondrion. Both yolk drops are surrounded by two membranes and contain within themselves micro-particles which closely resemble the cytoplasmic micro-particles. B, type B yolk drop; o.m., outer membranes; m., mitochondrion; y.m.p., yolk micro-particles; c.m.p., cytoplasmic micro-particles. Osmic and methacrylate. Magnification: × 28,000.

Fig. N. Type B yolk drop. The heavily osmiophilic granules are indicated with arrows. Osmic and methacrylate. Magnification: × 74,000.

Fig. O. Presumptive neural-plate ectoderm of a primitive streak stage blastoderm (see Textfig. 1A). Yolk drops are present and cytoplasmic micro-particles are sparse. A, type A yolk drop; B, type B yolk drop; c.m., cell membrane; n., nucleus; m., mitochondrion; c.m.p., cytoplasmic micro-particles. Osmic and methacrylate. Magnification: × 11,000.

Fig. P. Section through the hind-brain of an embryo at the stage of 11 pairs of somites (see Text-fig. 1 c, D). Yolk drops have disappeared and the number of cytoplasmic micro-particles has increased, n., nucleus; c.m., cell membrane; c.m.p., cytoplasmic micro-particles. Osmic and methacrylate. Magnification: × 11,000.