Book - The development of the chick (1919) 7
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Part II The Forth Day to Hatching, Organogeny, Development of the Organs
Chapter VII The External Form of the Embryo and the Embryonic Membranes
I. The External Form
General. The development of the external form of the embryo is conditioned by the order of development of the organs. The early form is thus given by the nervous system, somites and viscera. The development of muscles, bones, limbs, etc., that define the form of the fowl, begins relatively late, and only gradu.ally conceals the outlines of the internal parts.
Fig. 121. — A. Embryo of 3 days' and 16 hours' incubation, x 5. B. Embryo of 5 days' incubation, x 5. (After Keibel and Abraham.)
Figs. 121 to 124 illustrate the development of the external form from three days sixteen hours to ten days, (three days sixteen hours) the form of the head the brain, eyes, and visceral arches. The cerv strongly marked. There is no neck. The heart protuberance immediately behind the head. The rounded swellings. In Fig. 121 B (five days one vical flexure is less marked; the enlargement of
In Fig. 121 A is defined by ical flexure is makes a large limb-buds are hour) the certhe mid-brain makes a more pronounced protuberance of the head in this region; the heart has retreated farther back into the thorax, and the neck is thus indicated. The main divisions of the limbs are beginning to appear. In Fig. 122 (seven days seven hours) there are marked changes: The cervical flexure is practically lost. The elevation of the head and retreat of the heart into the thorax have produced a well-marked neck. The upper portion of the first visceral cleft alone is visible as the external auditory meatus; the other visceral arches and clefts have practically disappeared, excepting the mandibular arch, forming the lower jaw. The abdominal viscera begin to protrude. Feather germs have appeared in definite tracts. In the next stage, Fig. 123 (eight days), the contours of the body are decidedly bird like; the fore-limbs are wing-like. The contours of the head are much smoother, and determined more by the development of the facial region and skull than by the brain. The protuberance of the ventral surface caused by the viscera is strongly marked. Fig. 124 finally shows a ten-day embryo.
Fig. 122. — Embryo of 7 days' and 7 hours' incubation x 5. (After Keibel and Abraham.)
FiG. 123. — Embryo of 8 days x 5. (After Keibel and Abraham.)
The embryonic development of the head depends on the changes in three important classes of organs, together with their supporting and skeletal structures and accessory parts: (a) the central nervous system, (6) the organs of special sense, and (c) the visceral organs, mouth and pharynx. The origin of all these parts has been considered, and it is proposed to take up here only the development of the external form of the head. The preceding section gives an account sufficient for our present purposes, except in the case of the facial region. At four days this region appears as follows (Fig. 125): the mouth is a large, ill-defined opening, bounded behind by the mandibular arches, at the side by the maxillary processes, and in front by the nasofrontal process, which is a broad projection below the cerebral hemispheres overhanging the mouth. On each side of the nasofrontal process are the olfactory pits, the cavities of which are continuous with the oral cavity. Lateral to the olfactory pits are the external nasal processes, abutting against the eye and separated from the maxillary process by the lachrymal groove. The portion of the naso-frontal process bounding the olfactory pits on the median sides may be called the internal nasal process.
Fig. 124. — Embryo of 10 days and 2 hours x 5. (After Keibel and Abraham.)
Fig. 125. — Head of an embryo of 4 days' incubation, from the oral surface (N. L. 6 mm.)
Ep., Epiphysis. Hem., Cerebral hemisphere. Hy., Hyoidarch. 1. nas. pr., Lateral nasal process. Md., Mandibular arch. Mx., Maxillary process, nas.fr., Naso-frontal process. Olf., Olfactory pit. Or., Oral cavity. Ph., Pharynx, v. A. 3, Third visceral arch.
During the fourth and fifth days a fusion is graduall}' formed between the internal nasal process on the one hand, and the external nasal and maxillary processes on the other (Fig. 126), thus forming a bridge across the open mouth of the olfactory pits and dividing the openings in two parts, one within the oral cavity, which becomes the internal nares or choanae, and one without, which becomes the external nares or nostrils. During the same time ihe whole naso-frontal process begins to project forward to form the tip of the upper jaw. The two mandibular arches have also fused in the middle line and begin to project forward to form the lower jaw. This projection of upper and lower jaw causes a great increase in the depth of the oral cavity (Fig. 148).
The upper jaw is thus composed of three independent parts: vdz., the median part formed from the naso-frontal process and the two lateral parts formed from the maxillary processes. The former becomes the intermaxillary and the latter the maxillary region.
Fig. 126. — Head of an embryo of about 5 days from the oral surface. (N. L. 8 mm.)
ch. F., Choroid fissure. E. L., Eye-lid (nictitating membrane), ex. nar., External nares. 1. Gr., Lachrymal groove. Other abbreviations as before.
II. Embryonic Membranes
The extension of the blastoderm over the surface of the yolk goes on very rapidly up to the end of the fourth day of incubation (Fig. 33), at which time there is left a small circumscribed area of uncovered yolk, that may be called the umbilicus of the yolk-sac, which remains uncovered for a long time. Its final closure is associated with the formation of the albumen-sac.
The splitting of the mesoblast of the blastoderm is never complete ; but on the contrary the undivided margin begins to thicken after the fourth day, and gradually forms a ring of connective tissue that surrounds the umbilicus of the yolk-sac (Figs. 128 and 129). When this ring closes, about the seventeenth day, it forms a mass of connective tissue uniting the yolk-sac and albmnen-sac. (See below.)
During the first few days of incubation the all^umen loses water rapidly, and becomes more viscid, settling, as this takes place, towards the yolk-sac umbilicus. Thus the amniotic sac containing the embryo lies above; beneath the amniotic sac comes the volk, and the main mass of the albumen lies towards the caudal end of the embryo (Figs. 128 and 129).
The allantois expands very rapidly in the extra-embryonic body-cavity, and the latter extends by splitting of the mesoblast into the neighborhood of the yolk-sac umbilicus. When the allantois in its expansion approaches the lower pole of the egg, it begins to wrap itself around the viscid mass of the albumen accumulated there. In so doing, it carries with it a fold of the chorion, as it must do in the nature of the case, and thus the albumen mass begins to be surrounded by folds of the allantois with an intervening layer of the duplicated chorion. These relations will be readily understood by an examination of the accompanying diagrams (Figs. 128 and 129). In this way an albumen-sac, which rapidly becomes closed, is established outside of the yolk-sac, and the two are united by the undivided portion of the mesoblast around the yolk-sac umbilicus. This connection is never severed, and in consequence the remains of the albumen-sac is drawn with the yolk-sac into the body-cavity towards the end of incubation.
The sero-amniotic connection, which persists throughout incubation, has an important effect on the general disposition of the embryonic membranes. It is formed, as we have seen, in the closure of the amnion, by the thickened ectoderm of the suture; this ectodermal connection is, however, absorbed and replaced on the fifth to the seventh days by a broad mesodermal fusion, which maintains a permanent connection between amnion and chorion. One important result of this relation is that the albumen-sac, which is formed by the duplication of the chorion, is prolonged by a tubular diverticulum to the sero-amniotic plate (see Figs. 128 and 129). The latter becomes perforated after the eleventh day, and there is thus direct communication between the albumen-sac and the amniotic cavity. Hirota states that, after this connection is estabUshed, the amniotic fluid coagulates in alcohol, "just like the fluid in the albumensac; owing, presumably, to the presence of albumen which has found its way through the perforations into the amniotic fluid." This observation is confirmed by Fiilleborn.
Figs. 127, 128, and 129. — Diagrams of the relations of the embryonic membranes of the chick, constructed from preparations, and from figures and descriptions of Duval, Hans Virchow, Hirota and Fulleborn. In these figures the ectoderm and entoderm are represented by plain lines: The mesoderm by a cross-hatched line or band. The yolk-sac is represented by broken parallel lines. In Fig. 127 the allantois is represented as a sac. In Figs. 128 and 129, where it is supposed to be seen in section, its cavity is represented by unbroken parallel lines. The stalk of the allantois is exaggerated in all the diagrams to bring out its connection with the embryo. The actual relations of the stalk are shown in Figures 33 and 82. Alb., Albumen. Alb. S., Albumen-sac. All., Allantois. All. 1., Inner wall of the allantois. All. C, Cavity of allantois. All. S., Stalk of allantois.
All. 4- Am., Fusion of allantois and amnion. Am., Amnion. Am. C,
Amniotic cavity. Chor., Chorion. C. T. R., Connective tissue ring. Ect.,
Ectoderm. E. E. B. C, Extra-embryonic body-cavity. Ent., Entoderm.
Mes., Mesoderm. S.-Am., Sero-amniotic connection. 8. Y. S. U., Sac of
the yolk-sac umbilicus. Umb., Umbilicus. V. M., Vitelline membrane.
Y. S. S., Septa of the yolk-sac.
Fig. 127. — Fourth day of incubation. The embryo is surrounded by the amnion which arises from the somatic umbilicus in front and behind; the sero-amniotic connection is represented above the tail of the embryo; it consists at this time of a fusion of the ectoderm of the amnion and chorion. The allantois is represented as a sac, the stalk of which enters the umbilicus behind the yolk-stalk; the allantois lies in the extra-embryonic body-cavity, and its mesoblastic layer is fused with the corresponding layer of the chorion above the embryo. The septa of the yolk-sac are represented at an early stage. The splitting of the mesoderm has progressed beyond the equator of the yolk-sac, and the undivided portion is slightly thickened to form the beginning of the connective tissue ring that surrounds the yolk-sac umbilicus. The ectoderm and entoderm meet in the zone of junction, beyond which the ectoderm is continued a short distance. The vitelline membrane is ruptured, but still covers the yolk in the neighborhood of the yolk-sac umbilicus. The albumen is not represented in this figure.
Fig. 128. — Ninth day of incubation. The yolk-sac umbilicus has become much narrowed; it is surrounded by the mesodermal connective tissue ring, and by the free edges of the ectoderm and entoderm. The vitelline membrane still covers the yolk-sac umbilicus and is folded into the albumen. The allantois has expanded around the amnion and yolk-sac and its outer wall is fused with the chorion. It has pushed a fold of the chorion over the sero-amniotic connection, into which the mesoderm has penetrated, and thus forms the upper fold of the albumen-sac. The lower fold of the albumen-sac is likewise formed by a duplication of the chorion and allantois; it must be understood that lateral folds are forming also, so that the albumen is being surrounded from all sides.
The stalk of the allantois is exaggerated so as to show the connection of the allantois with the embryo; it is supposed to pass over the amnion, and not through the cavity of the latter, of course.
The part of the wall of the allantois that fuses with the chorion may be called the outer wall; the remainder of the sac of the allantois constitutes the inner wall. The distal intermediate part of the allantois is specialized with the chorion as the wall of the albumen-sac.
Fig. 129. — Twelfth day of incubation. The conditions represented in Fig. 128 are more advanced. The albumen-sac is closing; its connection with the cavity of the amnion by w^ay of the sero-amniotic connection will be obvious. The inner wall of the allantois has fused extensively with the amnion. The umbilicus of the yolk-sac is much reduced, and some yolk protrudes into the albumen (sac of the yolk-sac umbilicus).
In the outer wall there are three layers, viz., an internal epithelial laA^er, formed by the entoderm of the allantois; a thick very vascular middle or mesodermal laver, formed bv fusion of the mesoblast of allantois and chorion; and a thin, outer, ectodermal layer derived from the chorion.
Rate of Growth of the Allantois
As the embryo lies on its left side, the allantois grows out on the right side of the embryo (Figs. 127 and 130 A) and unites with the chorion about the one hundredth hour. It then spreads rapidly as a flattened sac over the embryo, increasing the extent of the fusion with the chorion, hence of its outer wall pari passu. At the end of the fifth day it covers more than half of the embryo (Fig. 130 A); at the end of the sixth day the embryo is entirely covered by the allantois (Fig. 130 B) ; at the end of the eighth day the alhmtois has covered half of the yolk-sac (Fig. 130 C). At the end of the ninth day, the formation of the albumen-sac is begun (Fig. 130 D). At the end of the eleventh day, the albumen-sac is practically closed at the lower pole. On the twelfth day, the albumen-sac is closed, and on the sixteenth day the contents are practically entirely absorbed.
Fig. 130. — Diagrams showing the relations of the allantois, represented by the tinted area, at different ages. (After
Hirota.) Alb., Albumen. Alb. S., Edge of albumen-sac.^ All. V., Allantoic vein. am. C, Amniotic cavity. S.-Am., Sero-ammotic connection. Y. 8., Yolk-sac. .
A. At 120 hours showing only the amniotic cavity and allantois X 2. . • -x ] 1
B. At 144 hours, showing only the amniotic cavity and allantois X 1.2. ^ n^i 1 ii ] 4 C. At 192 hours; the entire yolk x .66. Ihe dotted outline represents the amniotic cavity. , . i. i, ii
D. At 214 hours. The entire egg after removal ot the shell, X .66. The allximen mass is at the left ; the albumen-sac is beginning to form.
Blood-supply of the Allantois
There are two allantoic (umbilical) arteries and one allantoic vein. (See Chap. XII.) Both arteries persist throughout the period of incubation, but the left is much the better developed. It passes out along the stalk of the allantois to the inner wall of the allantoic sac, where it divides in two strong branches, one running cephalad and the other caudad to the margins of the sac where they pass over to the outer wall; The allantoic vein runs in the inner wall and passes over to the outer wall near the sero-amniotic connection. Both arteries and veins inhibit the expansion of the allantoic sac where they surround the margin; but the vein has by far the greatest effect, as its action is supplemented by the sero-amniotic connection. Thus indentations, gradually growing deeper, are established along the margins of the allantoic sac, and the outgrowth of the latter on each side of the indentations produce overlapping lobes (Figs. 130 C and D).
The arrangement of the smaller vessels and capillaries is very different in the outer and inner walls. In the outer wall the arteries and veins branch and interdigitate in the deeper portions of the mesoblast, and end in an extraordinarily finemeshed capillary netw^ork situated immediately beneath the thin ectoderm. "The capillaries form such narrow meshes, and have relatively so wide a lumen, that they can be compared only with those of the lungs of higher animals, and of the choroidea of the eye; indeed, instead of describing it as a vascular network embedded in tissue, one could as well describe it as a great blood-sinus interrupted by strands of tissue" (FiiUeborn.) This capillary network of the outer wall constitutes the respiratory area of the allantois. At the margins it passes gradually into the incomparably wider meshed capillary network of the inner wall. An extensive system of lymphatics is developed, l^oth in the outer and inner walls of the allantois, accompanying all the blood-vessels, even to their ultimate terminations.
Structure of the Allantois
(1) Inner wall. The inner wall of the allantois consists primarily of two layers, an inner entodermal and outer mesodermal layer. The latter soon becomes differentiated into two layers, an external, delicate, limiting layer of flat polygonal cells, with interlocking margins, and an intermediate layer of star-shaped cells embedded in a homogenous mucous ground substance. Parts of the inner wall become extremely thin, and in these regions the intermediate layer may become entirely absent. Elsewhere, particularly around the larger arteries and veins, the intermediate layer may attain considerable thickness. The entoderm becomes reduced to a layer of flat, interlocking cells. On the eighth day, spindleshaped muscle cells begin to appear in the mesoderm of the inner wall, and undergo rapid increase in numbers. Their distribution is somewhat irregular; in certain places they may even form several layers, and in others are practically wanting.
On the seventh day the inner wall of the allantois begins to fuse Avith the amnion in the neighborhood of the sero-amniotic connection, and this fusion rapidly extends over the area of contact between the two membranes. Within the area of fusion the muscle lavers of the allantois and amnion mutuallv reinforce each other, and in places no boundary can be found between them (Fiilleborn). But during the latter half of incubation the musculature of the fused area of allantois and amnion degenerates almost completely.
Towards the end of incubation, part of the inner wall of the allantois fuses also with the yolk-sac, and is therefore carried with the latter into the body-cavity of the chick.
(2) The Outer Wall of the Allantois. As already noted, the outer wail of the allantois fuses with the chorion. The compound membrane, which is respiratory in function, must be considered, therefore, as one. Over the entire respiratory area the ectoderm, belonging primarily to the chorion, which is elsewhere two layers of cells in thickness, becomes reduced to an exceedingly thin layer in direct contact with the walls of the capillaries internally and the shell membrane externally. According to Fiilleborn, the ectoderm cannot be distinguished as a separate layer in the latter half of incubation, and the capillaries appear to be in immediate contact with the shell-membrane. No muscular tissue appears to develop in the outer wall of the allantois.
(3) The Albumen-sac. The allantois in the course of its expansion over the embryo, between amnion and chorion, reaches the sero-amniotic connection; it must then either divide and ffrow round on eacli side of tlie connection, or evaginate the chorion above the connection and carry it as an overlapping fold on bej'Ond. The latter is what actually happens, and there is established as a consequence an overlapping fold of the chorion containing an extension of the allantois (Fig. 128); the space beneath this fold terminates, naturally, at the sero-amniotic connection. In the meantime the cleavage of the mesoblast has separated chorion and yolk-sac as far as the neighborhood of the volk-sac umbilicus, where the viscid albumen has accumuiated. The latter is situated not opposite to the yolk-stalk, but near the posterior pole of the yolk-sac, with reference to the embryo, i.e., usually towards the narrow end of the shell. Now the allantois growing around the yolk-sac from all sides reaches the neighborhood of the albumen and enters an evagination of the chorion that wraps itself around the albumen, thus initiating the formation of a double sac of the chorion enfolding the albumen and containing between its two layers an extension of the allantois. The latter is therefore separated everywhere from the albumen by the thickness of the chorion. The suj^erior fold of the albumen-sac is the same fold that overgrows the sero-amniotic connection, and the albumen-sac is therefore prolonged beneath this fold to the sero-amniotic connection itself, which, as we have seen, becomes perforated, thus admitting albumen into the amniotic cavitv.
The ectoderm lining the albumen-sac is two-layered, and the cells next the albumen tend to be cubical or swollen, and frequently vesicular, owing apparently to absorption of albumen. In the neighborhood of the yolk-sac umbilicus, papilla-like projections of the ectoderm into the albumen are common (Fig. 129). But these do not occur over the remainder of the albumen-sac of the chick, as described by Duval for the linnet; nor do they possess a mesodermal core.
Prior to the union of the mesoderm over the yolk-sac umbilicus, the yolk forms a hernia-like protrusion into the albumensac (sac of the yolk-sac umbilicus, see Fig. 129), which is, hoAvever, retracted as the mesoderm ring closes over the yolk-sac umbilicus. The vitelline membrane ruptures at an early period of the incubation over the embryonic pole and gradually slips down over the yolk, and is finally gathered together in the albumen-sac.
(4) The allantois also serves as a reservoir for the secretions of the mesonephros, and subsequently the permanent kidney, which reach it by way of the cloaca and neck of the allantois. The fluid part of the embryonic urine is absorbed, but the contained salts are deposited in the walls and cavity of the allantois. If the connection between the Wolffian ducts and cloaca be interrupted, the former become enormously extended by the secretions of the mesonephros.
The yolk-sac is established in the manner already described; it is constituted by the extra-embryonic splanchnopleure, and is permanently united to the intestine by the yolk-stalk. A narrow lumen remains in the stalk of the yolk-sac throughout, and even after, incubation, but the yolk does not seem to pass through it into the intestinal cavity. The walls of the yolk-sac, excepting the part derived from the pellucid area, are lined with a special glandular and absorbing epithelium, which digests and absorbs the yolk and passes it into the vitelline circulation, through which it enters the hepatic portal circulation and comes under the influence of the hepatic cells. The yolk-sac is thus the primary organ of nutrition of the embryo, and it becomes highly elaborated for the performance of this function. Contrary to the statements found in many text-books, it does not reach its maximum development until the end of incubation. Throughout incubation it steadily increases in complexity and efficiency so as to provide for the extremely rapid growth of the embryo.
The functions of the yolk-sac manifestly require a large surface area, which is provided for by foldings of the wall projecting into the yolk. At the height of its development the inner surface of the yolk-sac is covered with numerous folds or septa projecting into the yolk, which are highest at the equator and decrease in both directions away from the equator. In general, these folds follow the direction of the main arteries, i.e., they run in a meridional direction, repeatedly bifurcating distally (Fig. 132). Moreover, each one is perforated by numerous stomata, and the yolk-sac epithelium covers all free surfaces, and a capillary network is found in every part. So far do they project into the interior towards the close of incubation, that those of opposite sides may be approximately in contact, and the cavity of the yolk-sac is thus broken up into numerous connecting compartments filled with yolk. The outer wall of the yolk-sac is smooth and not involved in the folds. The beginning of the folds of the yolk-sac may be found at the time of appearance of the vascular area of the blastoderm, and they develop pari passu, with the vessels of the yolk-sac (Fig. 131).
Fig. 131 shows the appearance of the folds at the stage of twelve somites. It is a view of the blastoderm from below,
Fig. 131. — Septa of the yolk-sac as seen on the lower surface of the blastoderm at the stage of 12 s. (After Hans Virchow.)
m. R., Marginal ridge of entoderm overlying the sinus terminalis.
drawn as an opaque object, and it shows the incipient folds of the yolk-sac in an arrangement that corresponds roughly, but not accurately, with that of the blood-islands, which lie in large part in the bases of the folds. The site of the vena terminalis is marked bv a circular fold of the entoderm. The folds of the volk-sac thus coincide in their distribution with the vascular area and are so limited at all times, being absent in the vitelline area. There is thus a close connection between the vitelline blood vessels and the folds of the yolk-sac, which will be considered more fully beyond.
The interior of the yolk-sac is lined with entoderm which differs in its structure in different regions: In the area pellucida the cells are flattened; in the vascular zone of the area opaca are found the columnar cells with swollen ends described previously. After the third or fourth day these are found filled with yellow fatty droplets, which give a yellow tone to the interior of the living yolk-sac, and which are so abundant in later stages as to render the layer perfectly opaque. These cells do not contain entire yolk-granules; apparently, then, the yolk-granules are digested before absorption in this region. In the region of the inner zone of the vitelline area, the entoderm is composed of several layers of large cells containing yolk-granules, constituting the germ-wall, and in the outer vitelline zone we come to the periblast. The germinal wall and inner zone of the vitelline area represent the formative region of the yolk-sac epithelium in the manner already described (Chap. \).
FiG. 132. — Part of the interior of the yolk-sac of a duck at the time of hatchng. In the upper part of the figure the septa are seen from the side showing the stomata. In the lower part they are seen on edge. Note the sinuous course of the arteries along the free edges of some of the septa. (After H. Virchow.)
Blood-vessels of the Yolk-sac. The development of the circulation in the yolk-sac may be divided into the following stages (following Popoff) :
- Indifferent network bounded peripherally by the vena terminalis, connected by two anterior vitelline veins with the heart; no arterial trunks.
- Origin of an arterial path in the network; the right anterior vitelline vein begins to degenerate.
- Origin of intermediate veins; the (left) posterior vein begins to develop.
- Development of collateral veins; further degeneration of the right anterior vein; complete formation of the posterior vein.
- Further branching; development of a rich venous network; the vena terminalis begins to degenerate.
- Definitive condition; development of a rich venous network in the folds or septa of the yolk-sac; anastomosis of vessels of the yolk-sac and allantois.
The changes can be followed only in outline. The earliest condition has been described in Chapters IV and V. Fig. 133 show^s a condition intermediate between stages 1 and 2 above. The network is entirely arterial, except towards the anterior end, i.e., the blood flows outwards away from the heart. It enters the vena terminalis and is returned by right and left anterior vitelline veins to the heart. The beginning of arterial trunks in the network is indicated particularly on the left side (right side of the figure). The connection of the arterial network with the dorsal aorta is still net-like.
Fig. 134 shows an advance of the same processes. The trunks of the vitelline arteries are better differentiated from the network, and the blood is still returned to the heart entirely by way of the vena terminalis and the right and left anterior vitelline veins, which have come in contact distally, circumscribing in their proximal parts the mesoderm-free area of the blastoderm. The beginning of the lateral vitelline veins is indicated, particularly on the right side (left of the figure).
Fig. 135 represents a great advance. The vitelline arteries arise from the dorsal aortse as single trunks, and branch in the vascular network, some of them reaching as far as the vena terminalis. The two anterior vitelline veins have fused in front, and the right anterior vein is reduced in size so that most of the blood reaches the heart through the left anterior vein. But the most striking change is the transformation of part of the vascular network into channels in which the blood flows towards the heart. Of these there may be recognized the following: 1. Intermediate veins arising from the vena terminalis at various places and gradually losing themselves centrally in the vascular network. 2. The vascular network immediatelv behind the embrvo has assumed a venous character and likewise a large part of the network immediately surrouncUng the embryo. 3. Lateral vitelline veins are beginning to develop from the anterior intestinal portal backwards.
Fig. 133. — Circulation in the embryo and the yolk-sac. Stage of about 16 s; from below. The vitelline arteries are beginning to differentiate out of the vascular network particularly on the loft side. (Observer's right.) Injected. (After PopolT.;
1, Marginal vein. 2, Region (jf venous network. 3, First and second aortic arches. 4 r, 4 1, Right and left anterior vitelline veins. 5, Heart. 6, Anterior intestinal portal. 1, Aorta?. 8, Vitelline arteries in process of differentiation. 9, Blood islands.
Fig. 134. — Circulation in the embryo and the yolk-sac at the stage of about 22 s, drawn from below. Note differentiation of branches of the vitelline arteries. Injected. (After Popoff.) 1 Marginal vein. 2, Region of venous network. 3, Carotid loop. 4 r, 4 1 iiitrht and left anterior vitelline veins. 5, Heart. 6, Anterior intestinal portal. 7, Dorsal aorta. S, Branches of vitelline arteries.
Fig. 136, representing the circulation at a stage of about 40 somites, shows the completion of the primary circulation in the yolk-sac. The vitelline arteries branch richly, and end in a capillary network; very few arterial branches reach the vena terminalis as such, and then only very fine ones. The vena terminalis itself is relatively reduced; the lateral vitelline veins have absorbed the network between themselves and the intermediate veins, which now appear as prolongations of the lateral veins. The right anterior vitelline vein has disappeared almost entirely and the posterior vitelline vein is well developed, emptying into the left lateral vein.
The lateral vitelline arteries and veins are superposed as far peripherally as the original intermediate veins, which lie between the arterial trunks. Wherever there is superposition of arteries and veins, the latter are superficial and the former deep in position as seen from above. The figure also shows the vascular network in the budding allantois, and some of the embryonic blood-vessels.
In the later stages of development the arteries are carried in by the septa of the yolk-sac and lie near their free edges; the veins, on the other hand, remain superficial in position. The terminal vein becomes progressively reduced in importance up to about the tenth day, and then gradually disappears as such, being taken into the terminal capillaries. After the tenth day the anterior and posterior vitelline veins decrease in importance and finally become almost unrecognizable. The lateral veins, on the other hand, increase in importance and return all of the blood to the embryo.
The rich network of venous capillaries in the septa of the yolk-sac is shown in Fig. 137. It lies immediately beneath the epithelium over the entire extent of the septa and forms loops along the free border. The arteries do not communicate directly with this network according to Popoff , and the course of the circulation from arteries to veins is not clearly described by this author.
The allantois fuses with the yolk-sac in the region of the yolk-sac umbilicus, and anastomoses arise between the veins of the allantois and those of the yolk-sac.
Ultimate Fate of the Yolk-sac
On the nineteenth day of incubation, the yolk-sac slips into the body-cavity through the umbilicus; which thereupon closes. The mechanism of this process is of considerable interest. The yolk-sac is still a voluminous organ, and equal to about one sixth the weight of the embryo. It is therefore inconceivable that it could be "drawn into" the body-cavity by means of its stalk, which has only the intestine for attachment. The process is much more complex and may be briefly described as follows: We have already seen that the inner wall of the allantois fuses with the amnion on the one hand; distally it is connected with the yolk-sac. Now this wall of the allantois is muscular, and it is probable that its contraction is the first act in the inclusion of the yolk-sac within the body-wall. It is aided in this, however, by the inner wall of the amnion, i.e., that part of the amnion arising from the umbilicus and not fused with the allantois. This part of the amnion surrounds the yolk-stalk, and is itself richly provided with muscle cells, forming a crossing and interlacing system. It is carried down and over the yolk-sac to about its equator by the allantois, and when the yolk-sac is half taken into the body-cavity, it reaches its distal pole and fuses there. Now if the egg be opened at this stage in the process and this wall of the amnion cut through, it contracts rapidly to a fraction of its former area (Virchow). It is apparent, then, that the tension of this membrane on the yolk-sac must exert a continuous pressure that tends to force it into the body-cavity. It is in this way, then, by contraction of the inner walls of the allantois and of the amnion, that the yolk-sac is pressed into the body-cavity.
The umbilicus is therefore closed b}- the mere act of inclusion of the yolk-sac, for the inner amniotic wall is attached on the one hand to the body-wall, and on the other to the distal pole of the yolk-sac. A minute opening is left in the center of the umbilical field, through which dried remnants of the inner wall of the allantois, w^hich is likewise attached to the distal pole of the yolk-sac, protrude for a short time. On the inner side the yolk-sac is attached to the umbilicus by its distal pole, and by its stalk to the intestine. The absorption of the yolk-sac then goes on with great rapidity, being reduced from a weight of 5.34 gr. twelve hours after hatching to 0.05 gr. on the sixth day after hatching, according to a series of observations of Virchow.
Fig. 135. — Circulation in the embryo and yolk-sac after 74 hours' incubation. Stage of about 27 s from below. Injected. (After Popoff.)
1, Marginal vein. 2 r, 2 1, Right and left anterior vitelline veins surrounding the mesoderm-free area. 8, Anterior intestinal ]:)ortal. 4, Intermediate veins connecting with the venous network centrally. 5, Right dorsal aorta. 6, Posterior \itelline vein in j)rocess of formation. 7, Vitelline arteries.
Note that the right anterior vitelline vein (2 r) is much atrophied.
Fig. 136. — Circulation in the embryo and yolk-sac of an embryo of about 40 s, showing the later development of the lateral and intermediate vitelline veins. Reduction of vena terminalis (marginal vein). Almost complete atrophy of the right anterior vein. Injected. (After Popoff.)
1, Marginal vein. 2 r, 21, Right and left anterior vitelline veins. 3, Arch of aorta. 4, Left posterior cardinal vein. or, 51, Right and left omphalomesenteric veins. 6, Aorta. 6 a, Left dorsal aorta. 7, Vitelline artery. 8, Posterior vitelline vein. 9, Vascular network in the allantois.
The amnion invests the embryo closely at the time of its formation, but soon after, fluid begins to accumulate w^ithin the amniotic cavity, which gradually enlarges so that the embryo lies within a considerable fluid-filled space, which increases gradually up to the latter part of the incubation, and then diminishes again, so that the embryo finally occupies most of the cavitv. The connections of the amnion with the chorion, and later with the allantois, albumen-sac, and yolk-sac, have been already described.
Muscle fibers appear in the walls of the amnion on the fifth or sixth day and gradually increase in number; though they subsequently degenerate over the area of fusion with, the allantois. They persist elsewhere, how^ever, and are active in the inclusion of the yolk-sac in the manner already described. Shortly after the appearance of the muscle fibers slow vermicular or peristaltic contractions of the amnion begin, and the embryo is rocked within the amniotic cavity. Apparently, adhesions are thus prevented, but they are sometimes formed and lead to various malformations of the embryo. In some cases the amnion fails to develop; in such cases, the embryo usually dies at a relatively early stage, though Dareste records an anamniotic embryo of thirteen days, apparently full of life and vigor.
The amnion apparently acts first as a protection against all mechanical shocks and jars which are taken up by the fluid; second, by protecting the embryo against the danger of desiccation; third, by protecting it against adhesions with the shellmembrane and embryonic membranes, and lastly by providing space for the expansion of the allantois and consequent increase of the respiratory surface. It also has secondary functions in the chick in connection with the absorption of the albumen and the inclusion of the yolk-sac. It will be readily understood, then, why anamniotic embryos usually do not develop far.
Hatching (after von Baer). About the fourteenth day the growing embryo accommodates itself to the form of the egg so as to he parallel to the long axis with its head usually towards the broad end near to the air-chamber. Sometimes, however, the embryo is turned in the reverse position (von Baer). The head is bent towards the breast, and is usually tucked under the right wing. Important changes preparatory to hatching take place on the seventeenth to the nineteenth days. The fluid decreases in the amnion. The neck acquires a double bend so that the head is turned forward, and, in consequence, the beak is towards that part of the membranes next to the air-chamber. The intestine is retracted completely into the body-cavity, and on the nineteenth day the yolk-sac begins to enter the bodycavity. On the twentieth day the yolk-sac is completely included, and practically all the amniotic fluid has disappeared. The chick now occupies practically all the space within the egg, outside of the air-chamber. The umbilicus is closing over. The ductus arteriosi begin to contract, so that more blood flows through the lungs. The external w^all of the allantois fused with the chorion still remains very vascular.
Now, if the chick raises its head, the beak readily pierces the membranes and enters the air-chamber. It then begins to breath slowly the contained air; the chick may be heard, in some cases, to peep within the shell two days before hatching, a sure sign that breathing has begun. But the circulation in the allantois is still maintained and it still preserves its respiratory function. When the chick makes the first small opening in the shell, which usually takes place on the twentieth day, it begins to breathe normally, and then the allantois begins to dry up and the circulation in it rapidly ceases. It then becomes separated from the umbilicus, and the remainder of the act of hatching is completed, usually on the twenty-first day.
Fig. 137. — Part of a septum of the yolk-sac. Injected. 20 days' incubation. The free edge is above. (After Popoff.) Ar., Artery. St., Stomata. V. an., Longitudinal anastomoses of venous network. V., vein.