External Features.--The early appearance of the optic grooves (Stage E) which give rise to the optic vesicles; the existence of five visceral arches in Stage J ; the formation of the amnion first at the hind end of the embryo; and the folding off of the head end of the embryo only, are the chief points to be noted. The enclosure of the front end of the primitive streak within the medullary fold; the formation of protovertebræ, chiefly from before backwards; the closure of the medullary groove; the appearance of three divisions of the brain, and the formation of the heart are also detailed.
The Epiblast.--The epiblast of the embryo (Stages E--G) becomes formed into a median thickened portion, the medullary plate, and into lateral portions which are formed of cubical cells and are continuous with the flattened epiblast cells which cover the vesicle. The closure of the medullary groove (Stages H and J) causes the union of the lateral epiblast which thus forms a continuous layer across the embryo. The medullary groove commences about the centre of the embryo, widening out into the sinus rhomboidalis behind and into the cephalic plate anteriorly. The optic grooves are formed one on each side of the middle line in the cephalic plate (figs. 4 and 16).
The Medullary Canal.--The closure of the medullary groove commences in the region of the first protovertebra during Stage G and proceeds anteriorly and posteriorly, and at the close of Stage J a complete canal is formed as far back as the last (fourteenth) protovertebra. The lateral walls of the canal thicken, and are converted into an hour-glass form in places. The migration of mesoblast (nutritive) cells into the walls of the canal is noted in Stages H and J.
The Brain.--The three divisions of the brain are indicated in Stage J, and a well-marked cranial flexure is then present. The infundibulum is just apparent at this stage in close connection with the front end of the alimentary canal and notochord (fig. 49).
The Optic Vesicles are formed from the optic grooves by the closure of the medullary canal. These organs first appear extremely early, but their development is soon checked, doubt-less in consequence of the habits of the adult animal.
The Ear in Stage J is merely indicated as a deep groove in a thickened mass of mesoblast on either side of the hind-brain.
The Cranial and Spinal Nerves are not described.
The Hypoblast may be divided into axial and peripheral portions. The peripheral hypoblast, a single layer of flattened cells, extends on all sides over the embryonic area during Stage E. The deepening of the medullary groove stretches these cells and flattens them still more, but the thickening of the lateral mesoblast forces the lateral hypoblast down, removes the strain, and its cells become rounded.
The Notochord is formed of axial hypoblast cells. In Stage c a mass of axial hypoblast cells are continuous with two lateral masses of mesoblast--derived from lateral hypoblast--and with the lateral hypoblast layer also. In Stages D and E the axial mass becomes isolated from the lateral mesoblast plates, and gradually decreases in size below the deepening medullary groove until in that portion where the groove is deepest, i. e. near the centre, a single layer of flattened cells is all that exist.
It does not, however, become reduced to this extent through out its length; at the posterior end it remains thickened, and by the ingrowth of the lateral portions the axial cells first form an arch and then a complete tube, which is the neurenteric canal and which communicates dorsally with the exterior and ventrally with the yolk-sac.
This tube is the homologue of the median dorsal diverticulum of the alimentary tract in Amphioxus, i. e. the structure which gives rise to the notochord of that animal, and it is noteworthy that in the Mole it disappears almost entirely before the notochord is formed.
The single layer of cells to which the greater part of the axial hypoblaat is reduced at the close of Stage D (NO. 8) again increases in bulk during Stages E to J, and gives rise to the notochord.
As was the case with the lateral hypoblast, the flattening of these cells and their increase in bulk appears to be due, first to the stretching effect of the rapidly deepening medullary groove, and secondly to the release from that strain caused by the depression of the lateral portions of the embryo.
The isolation of the notochord first occurs in the region of the first protovertebra during Stage G, and extends anteriorly and posteriorly during Stages H and J.
The isolation is caused by the ingrowth of the lateral hypoblast below the axial cells, and the latter are isolated either as a solid band or rod, although a lumen may here and there appear in it afterwards.
At the close of Stage J the notochord is completely separated from the hypoblast, except at two points, viz. at the anterior end, where it is connected with the hypoblast and epiblast, where these two layers fuse to form the mouth, and posteriorly where it is joined to both epiblast, hypoblast, and mesoblast, at the front end of the primitive streak (figs. 49 and 50).
The origin of the notochord and the manner of its isolation appear to be sufficient reason to regard it as entirely homologous with the notochord of Amphioxus.
For a review of other opinions on this point I would refer to a discussion in my former paper (No. 8)
The hooked anterior end of the notochord is due to its origin from the front wall of the fore-gut. Its close approximation to the fore-brain is noted.
The relatively small size of the notochord to the nervous system in the Mole is pointed out, and it is suggested the early development of the latter is the cause of the check administered to the growth of the former, a check from which it appears never entirely to recover.
The Alimentary Canal first appears in Stage D as a short tubular diverticulum, projecting below the cephalic plate nearly to the anterior end of the embryo. The tube enlarges and extends backwards during the progress of the folding off of the embryo during Stages E to J, and the cranial flexure causes a ventral enlargement, which is somewhat posterior to the original anterior diverticulum.
The mouth and the visceral clefts are not formed at the close of Stage J, but the epiblast and hypoblast have fused at the point where the mouth will eventually be formed, and several lateral outgrowths from the now widened fore-gut exist; in the case of one of these, the anterior one, the hypoblast has reached the epiblast, and the two layers are partially fused at that point.
The mouth is formed at the apex of a Y-shaped groove, the diverging limbs of which are directed forwards j these grooves are the anterior border of the first visceral arch. The primary anterior diverticulum would indicate the existence primitively of a terminal mouth, while the two grooves, at the junction of which the mouth is formed, would suggest a paired origin for the existing mouth of the animal.
The Mesoblastic Somites and Body Cavity.--The lateral plates of mesoblast are split horizontally into somatic and splanchnic layers, but the split is not actually carried through both peripheral and axial portions of the plates, being merely indicated in Stage E in the axial portion. The mesoblast of the head also is not split, and ho cavity is formed there.
Protovertebræ are formed and the axial and peripheral portions of the mesoblast plates are separated from one another by the intermediate cell mass.
A cavity appears in the protovertebrse, Stage E, which still exists at the close of Stage J.
The formation of the muscle-plate commences at Stage H from the outer layer of cells of the protovertebra, but during Stage J, in the three anterior protovertebræ, a second row of cells derived from the inner (vertebral) portion of the somite, takes part in its formation, the two rows being continuous with one another at their dorsal and ventral ends.
The muscle-plates are first formed anteriorly. The outer cells of the muscle-plate in Stage s are prolonged into fine processes, which are connected with the overlying epiblast cells, and constitute voluntary muscular fibres (fig. 51). Certain of the cells of the inner layer are also differentiated into elongated muscular fibres.
I would further remark the mesoblast and epiblast cells in front of the primitive streak appear always to be connected together by processes.
The Pericardial Cavity only commences to form during Stage J, and is not at the close of that stage entirely separated from the remainder of the body cavity.
The Primitive Streak has the same relations in Stages E and F as in Stage D, except that the medullary folds grow backwards round its front end. The neurenteric canal disappears, but its original position is indicated by the fusion of the germinal layers at the front end of the primitive streak.
The mesoblast of the primitive streak does not give rise to the mesoblast of the body of the embryo in front of the primitive streak, in my opinion, but extends backwards and outwards and forms the wall of the allantois. The anus is formed in the middle of the primitive streak.
The Amnion is first formed at the hind end and from thence extends forwards. This portion of the amnion is formed of a double fold of somatopleure; the epiblast of the outer fold unites with the epithelium of the uterus. The anterior fold of the amnion, however, is formed only of epiblast and hypoblast, and has been called by van Beneden and Ch. Julin, who first described this structure, the "pro-amnion."
The Allantois commences in Stage F as a short wide diverticulum projecting upwards and backwards into the primitive streak. This diverticulum enlarges during Stages G to J ; it is lined with hypoblast cells (figs. 35 and 50).
The Arterial System.--The dorsal aortsæ commence in Stage F, and remain double until after Stage J; they are connected with the heart by a single pair of aortic arches during Stages H and J, and give off vitelline arteries at their posterior end. Internal carotid arteries and vertebral arteries are formed, and it is from the latter of these vessels the mesoblast cells are derived which migrate into the walls of the neural canal.
The Venous System is very slightly developed. Vessels are to be seen in the splanchnopleure over the yolk-sac at an early date, but vitelline veins connected with the heart are not seen until Stage H. TWO short anterior cardinal veins are present in Stage J, and traces of two posterior cardinals, but nothing more.
The Heart, which is formed of two tubes widely asunder in Stage E, is composed of a single tube for a short distance in Stage H, and is somewhat longer, but still straight and without sign of division into chambers at the close of Stage J. The thickened splanchnic mesoblast which gives rise to the heart, splits into two layers at an early age. The outer of these layers forms the outer wall of the heart, the inner the flattened epithelium of the cavity of the heart.
When the heart enlarges, as it does rapidly, a wide space exists between these two layers, but they are connected together by exceedingly fine processes of their cells which stretch across the space.
The Blood-Corpuscles appear to be formed from stellate mesoblast cells directly.
In conclusion, I may mention that I propose eventually to follow the further development of the organs of the Mole, one by one, and in doing so, to pay more attention to the researches of other investigators than has appeared to me advisable in the present paper.