Illustration: Figure 121bIn L the splanchnopleure has completely invested the yolk-sack, but at the lower pole of the yolk is still continuous with that peripheral remnant of the somatopleure now called the serous membrane. In other words, cleavage of the mesoblast has been carried all round the yolk (ys) except at the very lower pole.In M the cleavage has been carried through the pole itself; the peripheral portion of the splanchnopleure forms a complete investment of the yolk quite unconnected with the peripheral portion of the somatopleure, which now exists as a continuous membrane lining the interior of the shell. The yolk-sack (ys) is therefore quite loose in the pleuroperitoneal cavity, being connected only with the alimentary canal (a´) by a solid pedicle.Lastly, in N the yolk-sack (ys) is shewn being withdrawn into the cavity of the body of the embryo. The allantois is as before, for the sake of simplicity, omitted; its pedicle would of course lie by the side ofysin the somatic stalk marked by the usual dotted shading.It may be repeated that the above are diagrams, the various spaces being shewn distended, whereas in many of them in the actual egg the walls have collapsed, and are in near juxtaposition.
In L the splanchnopleure has completely invested the yolk-sack, but at the lower pole of the yolk is still continuous with that peripheral remnant of the somatopleure now called the serous membrane. In other words, cleavage of the mesoblast has been carried all round the yolk (ys) except at the very lower pole.In M the cleavage has been carried through the pole itself; the peripheral portion of the splanchnopleure forms a complete investment of the yolk quite unconnected with the peripheral portion of the somatopleure, which now exists as a continuous membrane lining the interior of the shell. The yolk-sack (ys) is therefore quite loose in the pleuroperitoneal cavity, being connected only with the alimentary canal (a´) by a solid pedicle.Lastly, in N the yolk-sack (ys) is shewn being withdrawn into the cavity of the body of the embryo. The allantois is as before, for the sake of simplicity, omitted; its pedicle would of course lie by the side ofysin the somatic stalk marked by the usual dotted shading.It may be repeated that the above are diagrams, the various spaces being shewn distended, whereas in many of them in the actual egg the walls have collapsed, and are in near juxtaposition.
Illustration: Figure 122Fig. 122. Diagrammatic longitudinal section through the axis of an embryo.The section is supposed to be made at a time when the head-fold has commenced but the tail-fold has not yet appeared.F.So.fold of the somatopleure.F.Sp.fold of the splanchnopleure;D.foregut.pp.pleuroperitoneal cavity between somatopleure and splanchnopleure;Am.commencing (head) fold of the amnion. For remaining reference lettersvidep.167.
Fig. 122. Diagrammatic longitudinal section through the axis of an embryo.The section is supposed to be made at a time when the head-fold has commenced but the tail-fold has not yet appeared.F.So.fold of the somatopleure.F.Sp.fold of the splanchnopleure;D.foregut.pp.pleuroperitoneal cavity between somatopleure and splanchnopleure;Am.commencing (head) fold of the amnion. For remaining reference lettersvidep.167.
Each fold is necessarily formed of two limbs, both limbs consisting of epiblast and a very thin layer of mesoblast; but in one limb the epiblast looks towards the embryo, while in the other it looks away from it. The space between the two limbs of the fold, as can easily be seen infig. 121, is really part of the space between the somatopleure and splanchnopleure; it is therefore continuous with the general space, part of which afterwards becomes the pleuroperitoneal cavity of the body, shaded with dots in the figure and marked (pp); so that it is possible to pass from the cavity between the two limbs of the amniotic folds into the cavity which surrounds the alimentary canal. When the several folds meet and coalesce together above the embryo, they unite in such a way that all their inner limbs unite to form a continuous inner membrane or sack, and all their outer limbs a similarly continuous outer membrane or sack. The inner membrane thus built up forms a completely closed sack round the body of the embryo, and is called the amnioticsack, oramnion proper(fig. 121, H, I,&c.,a), and the fluid which it afterwards contains is called the amniotic fluid, orliquor amnii. The space between the inner and outer sack is, from the mode of its formation, simply a part of the general cavity found everywhere between somatopleure and splanchnopleure. The outer sack over the embryo lies close under the vitelline membrane, and the cavity between it and the true amnion is gradually extended over the whole yolk-sack.
The actual manner in which the amniotic folds meet is somewhat peculiar (His and Kölliker). The head-fold of the amnion is the earliest formed, and completely covers over the head before the end of the second day. The side and tail folds are later in developing. The side-folds finally meet in the dorsal line, and their coalescence proceeds backwards from the head-fold in a linear direction, till there is only a small opening left over the tail. This also becomes closed early on the third day.
The allantois[67]is essentially a diverticulum of the alimentary tract into which it opens immediately in front of the anus. Its walls are formed of splanchnic mesoblast with blood-vessels, within which is a lining of hypoblast. It becomes a conspicuous object on the third day of incubation, but its first development takes place at an earlier period, and is intimately connected with the formation of the posterior section of the gut.
At the time of the folding in of the hinder end of the mesenteron the splitting of the mesoblast into somatopleure and splanchnopleure has extended up to the border of the hinder division of the primitive streak. As has been already mentioned, the ventral wall of the postanal section of the alimentary tract is formed by the primitive streak. Immediately in front of this is the involution which forms the proctodæum; while the wall of the hindgut in front of the anus owes its origin to a folding in of the splanchnopleure.
The allantois first appears as a protuberance of the splanchnopleure just in front of the anus. This protuberance arises, however, before the splanchnopleure has begun to be tucked in so asto form the ventral wall of the hindgut; and it then forms a diverticulum (fig. 123A,All) the open end of which is directed forward, while its blind end points somewhat upwards and towards the peritoneal space behind the embryo.
Illustration: Figure 123Fig. 123. Two longitudinal sections of the tail-end of an embryo Chick to shew the origin of the allantois. A at the Beginning Of The Third Day; B at the Middle of the Third Day.(After Dobrynin.)t.the tail;m.the mesoblast of the body, about to form the mesoblastic somites;x´.the roof ofx´´.the neural canal;Dd.the hind end of the hindgut;So.somatopleure;Spl.splanchnopleure;u.the mesoblast of the splanchnopleure carrying the vessels of the yolk-sack;pp.pleuroperitoneal cavity;Df.the epithelium lining the pleuroperitoneal cavity;All.the commencing allantois;w.projection formed by anterior and posterior divisions of the primitive streak;y.hypoblast which will form the ventral wall of the hindgut;v.anal invagination;G.cloaca.
Fig. 123. Two longitudinal sections of the tail-end of an embryo Chick to shew the origin of the allantois. A at the Beginning Of The Third Day; B at the Middle of the Third Day.(After Dobrynin.)t.the tail;m.the mesoblast of the body, about to form the mesoblastic somites;x´.the roof ofx´´.the neural canal;Dd.the hind end of the hindgut;So.somatopleure;Spl.splanchnopleure;u.the mesoblast of the splanchnopleure carrying the vessels of the yolk-sack;pp.pleuroperitoneal cavity;Df.the epithelium lining the pleuroperitoneal cavity;All.the commencing allantois;w.projection formed by anterior and posterior divisions of the primitive streak;y.hypoblast which will form the ventral wall of the hindgut;v.anal invagination;G.cloaca.
As the hindgut becomes folded in the allantois shifts its position, and forms (figs.123B and124) a rather wide vesicle lying immediately below the hind end of the digestive canal, with which it communicates freely by a still considerable opening; its blind end projects into the pleuroperitoneal cavity below.
Still later the allantois grows forward, and becomes a large spherical vesicle, still however remaining connected with the cloaca by a narrow canal which forms its neck or stalk (fig. 121G,al). From the first the allantois lies in the pleuroperitoneal cavity. In this cavity it grows forwards till it reaches the front limit of the hindgut, where the splanchnopleure turns back to enclose the yolk-sack. It does not during the third day project beyond this point; but on the fourth day begins to pass out beyond the body of the chick, along the as yet wide space between thesplanchnic and somatic stalks of the embryo, on its way to the space between the external and internal folds of the amnion, which it will be remembered is directly continuous with the pleuroperitoneal cavity (fig. 121K). In this space it eventually spreads out over the whole body of the chick. On the first half of the fourth day the vesicle is still very small, and its growth is not very rapid. Its mesoblast wall still remains very thick. In the latter half of the day its growth becomes very rapid, and it forms a very conspicuous object in a chick of that date (fig. 118,Al). At the same time its blood-vessels become important. It receives its supply of blood from two branches of the iliac arteries known as the allantoic arteries[68], and the blood is brought back from it by two allantoic veins which run along in the body walls (fig. 119) and after uniting into a single trunk fall into the vitelline vein close behind the liver.
Illustration: Figure 124Fig. 124. Diagrammatic longitudinal section through the posterior end of an embryo Bird at the time of the formation of the Allantois.ep.epiblast;Sp.c.spinal canal;ch.notochord;n.e.neurenteric canal;hy.hypoblast;p.a.g.postanal gut;pr.remains of primitive streak folded in on the ventral side;al.allantois;me.mesoblast;an.point where anus will be formed;p.c.perivisceral cavity;am.amnion;so.somatopleure;sp.splanchnopleure.
Fig. 124. Diagrammatic longitudinal section through the posterior end of an embryo Bird at the time of the formation of the Allantois.ep.epiblast;Sp.c.spinal canal;ch.notochord;n.e.neurenteric canal;hy.hypoblast;p.a.g.postanal gut;pr.remains of primitive streak folded in on the ventral side;al.allantois;me.mesoblast;an.point where anus will be formed;p.c.perivisceral cavity;am.amnion;so.somatopleure;sp.splanchnopleure.
Before dealing with the later history of the fœtal membranes, it will be convenient to complete the history of the yolk-sack.
Yolk-Sack. The origin of the area opaca has already been described. It rapidly extends over the yolk underneath the vitelline membrane; and is composed of epiblast and of thehypoblast of the germinal wall continuous with that of the area pellucida, which on the fourth day takes the form of a more or less complete layer of columnar cells[69]. Between the epiblast and hypoblast there is a layer of mesoblast, which does not extend as far as the two other layers. The yolk is completely surrounded by the seventh day.
Illustration: Figure 125Fig. 125. Diagram of the circulation of the Yolk-Sack at the end of the third day of incubation.H.heart;AA.the second, third and fourth aortic arches; the first has become obliterated in its median portion, but is continued at its proximal end as the external carotid, and at its distal end as the internal carotid;AO.dorsal aorta;L.Of.A.left vitelline artery;R.Of.A.right vitelline artery;S.T.sinus terminalis;L.Of.left vitelline vein;R.Of.right vitelline vein;S.V.sinus venosus;D.C.ductus Cuvieri;S.Ca.V.superior cardinal vein;V.Ca.inferior cardinal vein. The veins are marked in outline and the arteries are black. The whole blastoderm has been removed from the egg and is supposed to be viewed from below. Hence the left is seen on the right, andvice versâ.
Fig. 125. Diagram of the circulation of the Yolk-Sack at the end of the third day of incubation.H.heart;AA.the second, third and fourth aortic arches; the first has become obliterated in its median portion, but is continued at its proximal end as the external carotid, and at its distal end as the internal carotid;AO.dorsal aorta;L.Of.A.left vitelline artery;R.Of.A.right vitelline artery;S.T.sinus terminalis;L.Of.left vitelline vein;R.Of.right vitelline vein;S.V.sinus venosus;D.C.ductus Cuvieri;S.Ca.V.superior cardinal vein;V.Ca.inferior cardinal vein. The veins are marked in outline and the arteries are black. The whole blastoderm has been removed from the egg and is supposed to be viewed from below. Hence the left is seen on the right, andvice versâ.
Towards the end of the first day blood-vessels begin to bedeveloped in the inner part of the mesoblast of the area opaca. Their development is completed on the second day; and the region through which they extend is known as the area vasculosa. The area vasculosa also grows round the yolk, and completely encloses it not long after the area opaca. The part of the blastoderm which thus encloses the yolk forms the yolk-sack. The splitting of the mesoblast gradually extends to the mesoblast of the yolk-sack, and eventually the somatopleure of the sack, which is continuous, it will be remembered, with the outer limb of the amnion, separates completely from the splanchnopleure; and between the two the allantois inserts itself. These features are represented infig. 121 E, K, andL.
The circulation of the yolk-sack is most important during the third day of incubation. The arrangement of the vessels during that day is shewn infig. 125.
The blood leaving the body of the embryo by the vitelline arteries (fig. 125,R.Of.A,L.Of.A), which are branches of the dorsal aortæ, is carried to the small vessels and capillaries of the vascular area, a small portion only being appropriated by the pellucid area.
From the vascular area part of the blood returns directly to the sinus venosus by the main lateral trunks of the vitelline veins (R.Of,L.Of), and so to the heart. During the second day these venous trunks join the body of the embryo considerably in front of, that is nearer, the head than the corresponding arterial ones. Towards the end of the third day, owing to the continued lengthening of the heart, the veins and arteries run not only parallel to each other, but almost in the same line, the points at which they respectively join and leave the body being nearly at the same distance from the head.
The rest of the blood brought by the vitelline arteries finds its way into the lateral portions of a venous trunk bounding the vascular area, which is known as the sinus terminalis,S.T., and there divides on each side into two streams. Of these, the two which, one on either side, flow backward, meet at a point about opposite to the tail of the embryo, and are conveyed along a distinct vein which, running straight forward parallel to the axis of the embryo, empties itself into the left vitelline vein. Thetwo forward streams reaching a gap in the front part of the sinus terminalis fall into either one, or in some cases two veins, which run straight backwards parallel to the axis of the embryo, and so reach the roots of the heart. When one such vein only is present it joins the left vitelline trunk; where there are two they join the left and right vitelline trunks respectively. The left vein is always considerably larger than the right; and the latter when present rapidly gets smaller and speedily disappears. After the third day, although the vascular area goes on increasing in size until it finally all but encompasses the yolk, the prominence of the sinus terminalis becomes less and less.
The fœtal membranes and the yolk-sack may conveniently be treated of together in the description of their later changes and final fate.
On the sixth and seventh days they exhibit changes of great importance.
The amnion, at its complete closure on the fourth day, very closely invested the body of the chick: the true cavity of the amnion was then therefore very small. On the fifth day fluid begins to collect in the cavity, and raises the membrane of the amnion to some distance from the embryo. The cavity becomes still larger by the sixth day, and on the seventh day is of very considerable dimensions, the fluid increasing with it. On the sixth day Von Baer observed movements of the embryo, chiefly of the limbs; he attributes them to the stimulation of the cold air on opening the egg. By the seventh day very obvious movements begin to appear in the amnion itself; slow vermicular contractions creeping rhythmically over it. The amnion in fact begins to pulsate slowly and rhythmically, and by its pulsation the embryo is rocked to and fro in the egg. This pulsation is probably due to the contraction of involuntary muscular fibres, which seem to be present in the attenuated portion of the mesoblast, forming part of the amniotic fold. Similar movements are also seen in the allantois at a considerably later period.
The growth of the allantois has been very rapid, and it forms a flattened bag, covering the right side of the embryo, and rapidly spreading out in all directions between the primitive folds of theamnion, that is, between the amnion proper and the false amnion or serous envelope. It is filled with fluid, so that in spite of its flattened form its opposite walls are distinctly separated from each other.
The vascular area has become still further extended than on the fifth day, but with a corresponding loss in the definite character of its blood-vessels. The sinus terminalis has indeed by the end of the seventh day lost all its previous distinctness; and the vessels which brought back the blood from it to the heart are no longer to be seen.
Both the vitelline arteries and veins now pass to and from the body of the chick as single trunks, assuming more and more the appearance of being merely branches of the mesenteric vessels.
The yolk is still more fluid than on the previous day, and its bulk has (according to von Baer) increased. This can only be due to its absorbing the white of the egg, which indeed is diminishing rapidly.
During the eighth, ninth, and tenth days, the amnion does not undergo any very important changes. Its cavity is still filled with fluid, and on the eighth day its pulsations are at their height, henceforward diminishing in intensity.
The splitting of the mesoblast has now extended to the outer limit of the vascular area,i.e.over about three-quarters of the yolk-sack. The somatopleure at this point is continuous (as can be easily seen by reference tofig. 121) with the original outer fold of the amnion. It thus comes about that the further splitting of the mesoblast merely enlarges the cavity in which the allantois lies. The growth of this organ keeps pace with that of the cavity in which it is placed. Spread out over the greater part of the yolk-sack as a flattened bag filled with fluid, it now serves as the chief organ of respiration. It is indeed very vascular and a marked difference may be observed between the colour of the blood in the outgoing and the returning vessels.
The yolk now begins to diminish rapidly in bulk. The yolk-sack becomes flaccid, and on the eleventh day is thrown into a series of internal folds, abundantly supplied by large venous trunks. By this means the surface of absorption is largely increased, and the yolk is more and more rapidly taken up by theblood-vessels, and in a partially assimilated condition transferred to the body of the embryo[70].
By the eleventh day the abdominal parietes, though still much looser and less firm than the walls of the chest, may be said to be definitely established; and the loops of intestine, which have hitherto been hanging down into the somatic stalk, are henceforward confined within the cavity of the abdomen. The body of the embryo is therefore completed; but it still remains connected with its various appendages by a narrow somatic umbilicus, in which run the stalk of the allantois and the solid cord suspending the yolk-sack.
The cleavage of the mesoblast is still progressing, and the yolk is completely invested by a splanchnopleural sack.
The allantois meanwhile spreads out rapidly, and lies over the embryo close under the shell, being separated from the shell membrane by nothing more than the attenuated serous envelope, formed out of the outer primitive fold of the amnion and the remains of the vitelline membrane. With this membrane the allantois partially coalesces, and in opening an egg at the later stages of incubation, unless care be taken, the allantois is in danger of being torn in the removal of the shell-membrane. As the allantois increases in size and importance, the allantoic vessels are correspondingly developed.
On about the sixteenth day, the white having entirely disappeared, the cleavage of the mesoblast is carried right over the pole of the yolk opposite the embryo, and is thus completed (fig. 121). The yolk-sack now, like the allantois which closely wraps it all round, lies loose in a space bounded outside the body by the serous membrane, and continuous with the pleuroperitoneal cavity of the body of the embryo. Deposits of urates now become abundant in the allantoic fluid.
The loose and flaccid walls of the abdomen enclose a space which the empty intestines are far from filling, and on the nineteenth day the yolk-sack, diminished greatly in bulk but still of some considerable size, is withdrawn through the somatic stalk into the abdominal cavity, which it largely distends. Outside the embryo there now remains nothing but the highly vascularallantois and the bloodless serous membrane and amnion. The amnion, whose fluid during the later days of incubation rapidly diminishes, is continuous at the umbilicus with the body-walls of the embryo. The serous membrane (or outer primitive amniotic fold) is, by the completion of the cleavage of the mesoblast and the withdrawal of the yolk-sack, entirely separated from the embryo. The cavity of the allantois, by means of its stalk passing through the umbilicus, is of course continuous with the cloaca.
When the chick is about to be hatched it thrusts its beak through the egg-membranes and begins to breathe the air contained in the air chamber. Thereupon the pulmonary circulation becomes functionally active, and at the same time blood ceases to flow through the allantoic arteries. The allantois shrivels up, the umbilicus becomes completely closed, and the chick, piercing the shell at the broad end of the egg with repeated blows of its beak, casts off the dried remains of allantois, amnion and serous membrane, and steps out into the world.
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[62]The presence of numerous nuclei in the germinal wall was, I believe, first clearly proved by His (No.132). I cannot however accept the greater number of his interpretations.[63]Further investigations in confirmation of this widely accepted statement are very desirable.[64]This does not appear to be the case with the anterior opening in Melopsittacus undulatus, though its relations are not clear from Braun’s description (No.120).[65]This nomenclature may seem a little paradoxical. But on reflection it will appear that so long as the embryo is simply extended on the yolk-sphere, the point where the ventral surface begins has to be decided on purely morphological grounds. That point may fairly be considered to be close to the junction of the medullary plate and primitive streak. To use a mathematical expression the sign will change when we pass from the dorsal to the ventral surface, so that in strict nomenclature we ought in continuing round the egg in the same direction to speak of passing backwards along the medullary, but forwards along the primitive streak. Thus the apparent hind end of the primitive streak is really the front end, andvice versâ. I have avoided using this nomenclature to simplify my description, but it is of the utmost importance that the morphological fact should be grasped. If any reader fails to understand my point, a reference to fig. 52 B will, I trust, make everything quite clear. The heart of Acipenser (ht) is there seen apparently in front of the head. It is of course really ventral, and its apparent position is due to the extension of the embryo on a sphere. The apparent front end of the heart is really the hind end, andvice versâ.[66]VideMoldenhauer, “Die Entwicklung des mittleren und des äusseren Ohres.”Morphologisches Jahrbuch,Vol.III.1877.[67]For details on the development of the allantois the reader is referred to the works of Kölliker (No.135), Gasser (No.127), and for a peculiar view on the subject Kupffer (No.136). In addition to these works he may refer to Dobrynin“Ueber die erste Anlage der Allantois.”Sitz. der k. Akad. Wien,Bd.64, 1871. E. Gasser,Beiträge zur Entwicklungsgeschichte d. Allantois,etc.[68]I propose to call these arteries and the corresponding veins the allantoic arteries and veins, instead of using the confusing term ‘umbilical.’[69]Further investigations are required as to the character of this layer.[70]For details on this subjectvideA. Courty,“Structure des Appendices Vitellins chez le Poulet.”An. Sci. Nat.Ser.III. Vol.IX.1848.
[62]The presence of numerous nuclei in the germinal wall was, I believe, first clearly proved by His (No.132). I cannot however accept the greater number of his interpretations.
[63]Further investigations in confirmation of this widely accepted statement are very desirable.
[64]This does not appear to be the case with the anterior opening in Melopsittacus undulatus, though its relations are not clear from Braun’s description (No.120).
[65]This nomenclature may seem a little paradoxical. But on reflection it will appear that so long as the embryo is simply extended on the yolk-sphere, the point where the ventral surface begins has to be decided on purely morphological grounds. That point may fairly be considered to be close to the junction of the medullary plate and primitive streak. To use a mathematical expression the sign will change when we pass from the dorsal to the ventral surface, so that in strict nomenclature we ought in continuing round the egg in the same direction to speak of passing backwards along the medullary, but forwards along the primitive streak. Thus the apparent hind end of the primitive streak is really the front end, andvice versâ. I have avoided using this nomenclature to simplify my description, but it is of the utmost importance that the morphological fact should be grasped. If any reader fails to understand my point, a reference to fig. 52 B will, I trust, make everything quite clear. The heart of Acipenser (ht) is there seen apparently in front of the head. It is of course really ventral, and its apparent position is due to the extension of the embryo on a sphere. The apparent front end of the heart is really the hind end, andvice versâ.
[66]VideMoldenhauer, “Die Entwicklung des mittleren und des äusseren Ohres.”Morphologisches Jahrbuch,Vol.III.1877.
[67]For details on the development of the allantois the reader is referred to the works of Kölliker (No.135), Gasser (No.127), and for a peculiar view on the subject Kupffer (No.136). In addition to these works he may refer to Dobrynin“Ueber die erste Anlage der Allantois.”Sitz. der k. Akad. Wien,Bd.64, 1871. E. Gasser,Beiträge zur Entwicklungsgeschichte d. Allantois,etc.
[68]I propose to call these arteries and the corresponding veins the allantoic arteries and veins, instead of using the confusing term ‘umbilical.’
[69]Further investigations are required as to the character of this layer.
[70]For details on this subjectvideA. Courty,“Structure des Appendices Vitellins chez le Poulet.”An. Sci. Nat.Ser.III. Vol.IX.1848.
The formation of the germinal layers in the Reptilia is very imperfectly known. The Lizard has been studied in this respect more completely than other types, and there are a few scattered observations on Turtles and Snakes.
The ovum has in all Reptilia a very similar structure to that in Birds. Impregnation is effected in the upper part of the oviduct, and the early stages of development invariably take place in the oviduct. A few forms are viviparous,viz.some of the blindworms amongst Lizards (Anguis, Seps), and some of the Viperidæ and Hydrophidæ amongst the Serpents. In the majority of cases, however, the eggs are laid in moist earth, sand,&c.Around the true ovum an egg-shell (of the same general nature as that in birds, though usually soft), and a variable quantity of albumen, are deposited in the oviduct. The extent to which development has proceeded in the oviparous forms before the eggs are laid varies greatly in different species.
The general features of the development (for a knowledge of which we are mainly indebted to Rathke’s beautiful memoirs), the structure of the amnion and allantois,&c.are very much the same as in Birds.
The Lizards will be taken as type of the class, and a few noteworthy points in the development of other groups will be dealt with at the close of the Chapter. The following description, taken in the main from my own observations, applies to Lacerta muralis.
The segmentation is meroblastic, and similar to that in Birds. At its close the resulting blastoderm becomes divided into two layers, a superficial epiblast formed of a single row of cells, anda layer below this several rows deep. Below this layer fresh segments continue for some time to be added to the blastoderm from the subjacent yolk.
Illustration: Figure 126Fig. 126. Sections through an embryo of Lacerta muralis represented in fig. 129.m.g.medullary groove;mep.mesoblastic plate;ep.epiblast;hy.hypoblast;ch´.notochordal thickening of hypoblast;ch.notochord;ne.neurenteric canal (blastopore). In E.nepoints a diverticulum of the neurenteric canal into the primitive streak.
Fig. 126. Sections through an embryo of Lacerta muralis represented in fig. 129.m.g.medullary groove;mep.mesoblastic plate;ep.epiblast;hy.hypoblast;ch´.notochordal thickening of hypoblast;ch.notochord;ne.neurenteric canal (blastopore). In E.nepoints a diverticulum of the neurenteric canal into the primitive streak.
The blastoderm, which is thickened at its edge, spreads rapidly over the yolk. Shortly before the yolk is half enclosed a small embryonic shield (area pellucida) makes its appearance near the centre of the blastoderm. The embryonic shield is mainly distinguished from the remainder of the blastoderm by the more columnar character of its constituent epiblast cells. It is somewhat pyriform in shape, the narrower end corresponding with the future posterior end of the embryo. At the hind end of the shield a somewhat triangular primitive streak is formed, consisting of epiblast continuous below with a great mass of rounded mesoblast cells, probably mainly formed, as in the bird, by a proliferation of the epiblast. To this mass of cells the hypoblast is also partially adherent. At the front end of the streak an epiblastic involution appears, which soon becomes extended into a passage open at both extremities, leading obliquely forwards through the epiblast to the space below the hypoblast. The walls of the passage are formed of a layer of columnar cells continuous both with epiblast and hypoblast. In front of the primitive streak the body of the embryo becomes first differentiated by the formation of a medullary plate; and at the same time there grows out from the primitive streak a layer of mesoblast, which spreads out in all directions between the epiblast andhypoblast. In the region of the embryo the mesoblast plate is stated by Kupffer and Benecke to be continuous across the middle line, but this appears very improbable. In a slightly later stage the medullary plate becomes marked by a shallow groove, and the mesoblast of the embryo is then undoubtedly constituted of two lateral plates, one on each side of the median line. In the median line the notochord arises as a ridge-like thickening of the hypoblast, which is continued posteriorly into the front wall of the passage mentioned above.
The notochord does not long remain attached to the hypoblast, and the separation between the two is already effected for the greater part of the length of the embryo by the stage represented infig. 129.Fig. 126represents a series of sections through this embryo.
Illustration: Figure 127Fig. 127. Diagrammatic longitudinal section of an embryo of Lacerta.pp.body cavity;am.amnion;ne.neurenteric canal;ch.notochord;hy.hypoblast;ep.epiblast of the medullary plate;pr.primitive streak. In the primitive streak all the layers are partially fused.
Fig. 127. Diagrammatic longitudinal section of an embryo of Lacerta.pp.body cavity;am.amnion;ne.neurenteric canal;ch.notochord;hy.hypoblast;ep.epiblast of the medullary plate;pr.primitive streak. In the primitive streak all the layers are partially fused.
In a section (A) through the trunk of the embryo a short way in front of the primitive streak, there is a medullary plate with a shallow groove (mg), well-developed mesoblastic plates (mep), already divided into somatic and splanchnic layers, and a completely formed notochord independent of the hypoblast (hy). In the next section (B), taken just in front of the primitive streak, the notochord is attached to the hypoblast, and the medullary groove is deeper; while in the section following (C), which passes through the front border of the primitive streak, the notochord and hypoblast have become fused with the epiblast. The section behind (D) shews the neurenteric passage leading through the floor of the medullary groove and through the hypoblast (ne). On the right side the mesoblastic plate has become continuous with the walls of the passage. The last section (E) passes through the front part of the primitive streakbehind the passage. The mesoblast, epiblast, and to some extent the hypoblast, are now fused together in the axial line, and in the middle of the fused mass is seen a narrow diverticulum (ne) which is probably equivalent to the posterior diverticulum of the neural canal in Birds (videp.164).
The general features of the stage will best be understood by an examination of the diagrammatic longitudinal section represented infig. 127. In front is shewn the amnion (am), growing over the head of the embryo. The notochord (ch) is seen as an independent cord for the greater part of the length of the embryo, but falls into the hypoblast shortly in front of the neurenteric passage. The neurenteric passage is shewn atne, and behind it is the front part of the primitive streak.
It is interesting to notice the remarkable relations of the notochord to the walls of the neurenteric passage. More or less similar relations are also well marked in the case of the goose and the fowl, and support the conclusion, deducible from the lower forms of Vertebrata, that the notochord is essentially hypoblastic.
The passage at the front end of the primitive streak forms the posterior boundary of the medullary plate, though the medullary groove is not at first continued back to it. The anterior wall of this passage connects together the medullary plate and the notochordal ridge of the hypoblast. In the stage represented infig. 126and129the medullary groove has become continued back to the opening of the passage, which thus becomes enclosed in the medullary folds, and forms a true neurenteric passage[71].
It will be convenient at this point to say a few words as to what is known of the further fate of the neurenteric canal, and the early development of the allantois. According to Strahl, who has worked on Lacerta vivipara, the canal gradually closes from below upwards, and is obliteratedbefore the completion of the neural canal. The hind end of the alimentary tract appears also to become a closed canal before this stage.