Book - Text-Book of the Embryology of Man and Mammals 11

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Hertwig O. Text-book of the embryology of man and mammals. (1892) Translated 1901 by Mark EL. from 3rd German Edition. S. Sonnenschein, London.

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The Foetal Membranes of Reptiles and Birds

As has already been stated, the course of development in all animals which do not deposit their eggs in water in Reptiles, Birds, and Mammals is unusually complicated, owing to the appearance of special egg-envelopes (embryonic or foetal membranes). Some of them, according to their origin, are to be referred to the extraembryonic area of the germlayers, and indeed to that part which in Fishes is employed for the yolk-sac. They arise from folds, which grow around the embryo while it is still small, and furnish a double envelope for it.

The egg-envelopes (embryonic membranes) of Reptiles and Birds, which exhibit almost identical conditions, and the consideration of which we shall take up first, are more simply constituted than those of Mammals. In the case of the former there are associated with the yolk-sac, in the possession of which they agree with the Amphibia and Fishes, three additional embryonic appendages, the amnion, the membrana serosa (or briefly serosa), and the allantois. They are partly laid down at an early period, at the time when the embryonic body is converted into tubes by the infolding of the germ-layers and is thereby constricted off from the yolk-sac.

The Chick shall again serve as a basis for our description.

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Fig. 124. Surface-view of the pellucid area of a blastoderm of a Chick of 18 hours, after BALFOUR.

In front of the primitive groove, pr, lies th e medullary furrow surrounded by the medullary folds. Immediately in front of these one sees a curved line, the head-fold, and in front of it a second curved line running concentric with it, the anterior fold of the amnion.

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The Amnion, the Serosa, and the Yolk-Sac

The amnion is a structure the appearance of which is recognisable remarkably early in the Chick. At the time when one recognises the semicircular head-fold at the anterior end of the incipient embryo (fig. 124), by the growth of which the head of the embryo is marked off, there is already present, at a short distance from it, a second fold running parallel to it. This is the anterior fold of the amnion, a product of the extra-embryonic part of the ectoderm and of the parietal mesoderm united with it. The two infol dings, which lie near to each other, have opposite directions (fig. 125). While the head-fold (F.So) advances with its margin toward the yolk, the anterior fold of the amnion (Am), separated from it by the marginal] groove, Vises externally above the plane of the blastoderm. At the time when the head is being formed, the amnion enlarges rather rapidly (Plate I., fig. 1 1 vaj), and grows over and around the head in a cap-like fold, the rim of which is directed backwards. At the end of the second day of incubation it already covers the anterior part of the head like a thin transparent veil, and is therefore called the cephalic sheath.

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Fig. 125. Diagrammatic longitudinal section through the axis of an embryo Bird, after BALFOUR. The section represents the condition when the head-fold is already formed, but the tail-fold is still wanting. F.So, Head-fold of the somatopleure ; F.Sp, head-fold of the splanchnopleure, forming at Sp the floor of the anterior part of the intestine. For the remaining references see fig. 122, p. 201. +++++++++++++++++++++++++++++++++++++++++

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Fig. 126. Diagrammatic longitudinal section through the posterior end of an embryo Chick at the time of the formation of the allantois, after BALFOUE. ep, me, hy, Outer, middle, and inner germ-layers ; ch, chorda ; SjJ.c, neural tube ; n.e, neurenteric canal ; p.a.g, post-anal gut ; pr, remains of the primitive streak folded toward the ventral side ; al, allantois ; an, point where the anus will be formed ; p.c, peri visceral cavity ; am, amnion ; so, somatopleure ; sp, splanchnopleure.

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In like manner, but at a somewhat later stage, there arise at the tail-end and at both sides of the embryo the posterior and lateral folds of the amnion. The posterior fold is still very inconspicuous even at the time when the head is covered with the veil-like pellicle (Plate I., fig. 11 hcif). It enlarges slowly, and under the name of caudal sheath covers over the posterior end of the body (fig. 126 am). The lateral folds of the amnion are elevated externally to the lateral marginal grooves (fig. 127 om), and project in the opposite direction from those lateral folds by the bending in of which the lateral and ventral walls of the embryo are produced. By this means the rim of the fold is carried farther and farther from the splanchnopleure (sp), which remains spread out flat over the yolk. In this way the extra-embryonic part of the body-cavity, or the cavity of the blastoderm (KOLLIKER), increases in extent in the vicinity of the embryo. When the lateral folds of the anmion have grown up to the dorsal surface of the embryo (Plate I., fig. 9 sqf), they begin, by the bending over of their edges medianwards, to form the so-called lateral sheaths.

Inasmuch as the folds of the amnion, which are called by special names, become, when they are in full development, continuous, and are only parts of a single ring-like fold, the embryo eventually becomes surrounded on all sides as though by a high wall. With further enlargement, the amniotic sheaths then bend together over the back of the embryo from in front and behind, and from the right and the left (Plate I., figs. 2, 3, and 10, of, vaf, haf), come together with their edges in the median plane, and then fuse with each other along a line, the amniotic suture, which closes from in front backwards (Plate I., fig. 10), except that at one very small place near the tail-end the closing is interrupted for a considerable time, and a small opening is preserved.

The fusion of the amniotic folds takes place in the same manner as the fusion of the medullary folds described on page 79. Each fold (Plate I., figs. 3 and 10) consists of two layers, an inner and an outer one, which are continuous at the margins of the folds, and are separated by a fissure, which is a portion of the extra-embryonic body-cavity. At the amniotic suture the corresponding layers of the folds of both sides fuse, and hand in hand with this a separation of the inner from the outer layers takes place (Plate I., fig. 4). As a result of this there have now arisen two envelopes over the back of the embryo, an inner and, an outer one, the amnion (A) and the serosa (S).

The amnion is the product of the inner layer of the folds (Plate I., fig. 10 ifb). It forms a sac which immediately after its origin is closely applied about the embryo, and which encloses a very small amniotic cavity filled with fluid.

The serous membrane (serosa), which is derived from the outer layer of the folds (afb, Plate I., fig. 10), lies as a very delicate transparent membrane closely applied to the amnion, and thus encloses the embryo in still another envelope.

If we now glance back at the conditions described in the previous chapter, and compare the development of Fishes with that of Reptiles and Birds, it is to be seen that a considerable complication has arisen in the case of the latter. Whereas in Fishes the extra-embryonic area of the somatopleure becomes exclusively the dermal yolk-sac, in Reptiles and Birds two sacs have arisen out of it by a process of folding. The influences producing this folding appear to be clear.

Since the egg is enclosed in firmly applied envelopes, the embryonic body, when it is formed by the folding together of the germ-layers, cannot rise from the yolk-sac ; it therefore comes to lie in a depression of the latter. There is the more reason for the occurrence of this because the embryo at the beginning of development is excessively small in comparison with the yolk, and because the yolk-layers immediately underlying it become liquefied and absorbed. With the sinking of the body into the yolk (Plate I., figs. 2 and 3), the parts which in Fishes become the simple dermal yolk-sac (Plate I., figs. 6 and 7) fold in around it on all sides as amniotic folds, and enclose it the more completely the deeper it sinks into the yolk.

The preceding account of the development of the aninion is made somewhat schematic in a single point. That is to say, the anterior fold of the aninion is developed so early, that the middle germ-layer has not yet been able to spread out as far as the anterior part of the embryonic area. The infolding, therefore, in this region involves only the outer and inner germ-layers, which are still closely united. This condition is changed somewhat later, when the middle germ-layer has grown into the region of the anterior fold of the amnion, and has there split into a visceral and a parietal layer. The process has not yet been followed out in detail in series of longitudinal sections. But at all events we must assume that the entoblast, which is united with the visceral middle layer, retracts from the anterior fold of the amnion and again spreads out flat, as is represented in diagrammatic figure 11 (Plate I.). In this manner the anterior amniotic fold, which in the meantime has become greatly enlarged, now consists of the outer germ-layer and the parietal middle layer, as is the case from the beginning with the subsequently arising posterior and lateral folds of the amnion.

We now have to enter still more particularly upon the further relations of amnion and seros;;.

Up to the end of embryonic development the amniotic sac remains in continuity with a small region on the ventral side of the embryo, which is called the dermal umbilicus. In figs. 3, 4, 5, and 10 (Plate I.) this place is indicated by means of a circular line (7m). Here the primitive layers of the body-wall are continuous wdth the corresponding layers of the amnion, as, for instance, the epidermis of the body with an epithelial layer lining the amniotic cavity. The dermal umbilicus of Reptiles and Birds corresponds therefore with the structure of the same name in embryo Fishes (Plate I., fig. 7 7m), for it is at this point that the dermal yolk-sac is continuous by means of its stem-like elongation with the walls of the belly. As in the Fishes, it surrounds an opening (Plate I., figs. 7 and 5 kn) which unites the portion of the body-cavity lying within the embryo (lk } ) with the extra-embryonic part lying between the embryonic membranes (lh 2 ). Furthermore, the stalk of the yolk-sac or vitelline duct, which is continuous with the embryonic intestine, and which is indicated in the above-mentioned figures of Plate I. by the small circle dn, passes through the opening.

The amniotic sac affords an additional special advantage to the embryos of Reptiles and Birds in that an albuminous saline fluid, the liquor arnnii, collects in its cavity. In it the delicate, easily injured embryo composed of plastic cells floats, as it were, and is able to move.

The amniotic sac is small at the beginning of its development, but enlarges with each day of incubation, since it keeps pace with the growth of the embryo and encloses a larger and larger amount of amniotic fluid.

At the same time its wall becomes contractile. Certain cells in its somatic mesoderm develop into contractile fibres, which in the Chick give rise to rhythmic movements from the fifth day of incubation onward. One can observe these while the egg-shell remains intact, if one holds the egg toward a source of blight light, and for this purpose makes use of the ooscope constructed by PREYER. In this manner it can be determined that the amnion executes about ten contractions in a minute, which, beginning at one pole, proceed to the opposite end, like the contractions of a worm. Thus the amniotic fluid is set in motion, and the embryo oscillates or rocks regularly from one end to the other. The rocking of the embryo, as PREYER expresses it, becomes more and more obvious in the later clays of incubation, since the contractions of the amnion become more energetic.

The serosa (S) is a wholly transparent, easily ruptured membrane, which is closely applied to the vitelline membrane. It consists of two thin cell-layers, which take their origin from the outer germ-layer and the parietal middle layer, and like them are distinguished by blue and red lines in the diagram. The serous membrane is originally present as a separate structure only in the region of the amnion and of the embryo (Plate I., fig. 4), as far as the body-cavity is formed in the middle germ-layer. It then enlarges to the same extent as the yolk becomes overgrown and as the vascular area extends farther downwards. Parietal and visceral middle layers separate more and more from each other, until finally (in the Chick toward the end of incubation) a separation results over the entire periphery of the yolksphere. Figs. 3, 4, and 5, Plate I., show stages in this process. In the last figure, which represents the condition on about the seventh day of incubation, the extra-embryonic part of the body-cavity has already become very considerable ; the serous envelope is, with the exception of a small place at the vegetative pole of the yolk, everywhere formed as a separate structure.

In connection with this the wall of the yolk-sac also becomes changed. Whereas at the beginning of the overgrowth it embraces for a considerable distance all the germ-layers, after the separation of the serosa it is composed exclusively of entoderm and the visceral middle layer.

EXPLANATION OF THE FIGURES ON PLATE I

Figs. 1-5 are diagrammatic representations of cross and longitudinal sections through the Hen's egg at different stages of incubation. They are intended to illustrate how the body of the Chick is developed out of the embryonic fundament, and how the yolk-sac, the arnnion, the serosa, and the allantois arise out of the extra-embryonic area of the germ-layers.

For the sake of clearness the embryonic fundament, and later the embryo, are represented much too large in relation to the yolk.

In order more easily to distinguish the different parts from one another different colors have been selected for them. The yolk is represented in yellow, the entoderm green, the outer germ-layer blue, and the middle germlayer, together with the rnesenchyme, red. The black dots indicate the limit to which the outer and inner germ-layers have grown over the yolk in the different stages ; the red dots mark the boundary for the time being of the middle germ-layer, which after the development of the blood-vessels ends in the sinus terminalis.

The references apply to all of tlie figures.

clc, Outer (jerm-lcnier (blue).

mw, Medullary ridges or folds.

N, Neural tube.

a/, Amniotic fld.

vaf, Jiaf, saf, Anterior, posterior, and lateral amniotic folds. A, Amnion. alt, Amniotic cavity. S, Serous membrane (Serosa). Jut, Dermal umbilicus. */, Lateral folds. Tcf\ kf", Head-fold ; afb, >fb, outer and inner limbs of fold. ik, Inner germ-layer (green), it/', Its margin of overgrowth. dr, Intestinal groove.

dg, Vitelline duct.

al, Allantois.

ds, Intestinal sac..

dn, Intestinal umbilicus.

rak, Miihlle germ-lai/er (red).

ink *, Parietal lamella of the same or parietal middle layer.

mk 2 , Visceral lamella of the same or visceral middle layer.

st, Lateral limit of the same, sinus terminalis, marginal vein.

dni, rm, Dorsal and ventral mesenteries.

Ih, Body-cavity. lh l , Embryonic, lh", extraembryonic part of the same.

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Fig. 1. Cross section through a Hen 8 egg on the second day of incubation.

The germ-layers are spread out flat over the yolk ; the middle one is less extensive than the other two. The first blood-vessels have developed, and terminate with the marginal vein (.?) at the edge of the middle germ-layer. One now distinguishes therefore the vascular area, which extends to the red dotted line (.<tf)> and external to it the yolk-area (flit), which terminates with the black dotted line (?/r), the edge of overgrowth of the outer and inner germ-layers.

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Fig. 2. Cross section through a Hen's egg on the third day of incubation.

The outer and inner germ-layers are spread out over half of the yolk. The yolk-area (dh} terminates with the black dotted line (ur), the edge of overgrowth.

The middle germ-layer, with the vascular area, which is now well developed, has also grown over the yolk as far as the line st (the sinus terminalis). In the middle germ-layer the body-cavity has become distinct in the embryonic region (Ut } ) and in its immediate vicinity (7/r), the parietal (mk ] ) and visceral middle layers (?7r; 2 ) having separated from each other.

The embryonic fundament -begins to be constricted off from the extraembryonic part by a process of folding and to constitute the trunk. The lateral folds (sf) have grown downwards for a certain distance, thus giving rise to the lateral walls of the trunk, whereas ventrally the body is still open. Corresponding to these lateral folds (./). the lateral intestinal folds (df) have arisen on the splanchnopleure, and bound the intestinal groove (dr).

The embryo in process of being constricted off has sunk into a depression of the more and more liquefied yolk, and becomes partly enveloped by the somatopleure of the extra-embryonic area of the germ-layers, the lateral folds of the amnion (af) having already encircled the sides of the embryonic body.

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Fig. 3 shows a longitudinal section through, the stage represented in cross section in Fig. 2. (Third day of incubation.) The head-end of the body is entirely constricted off from the blastoderm. It encloses the cephalic portion of the intestine (Kopfdarrnhb'hle). The tailend is only slightly differentiated. The anterior fold of the amnion (vaf) has invested the head, the posterior fold (//#/) the tail (cephalic sheath, caudal sheath).

The middle of the trunk is still wide open ventrally. The place where the body-wall passes over into the folds of the amnion, and which is indicated in the diagram by the ring hn, is called the dermal umbilicus.

The splanchnopleure has become closed into a tube anteriorly and posteriorly (the cephalic and pelvic portions of the intestinal cavity) ; in the middle the tube is still open ventrally, and by means of the vitelline duct (dg} is continuous with the yolk-sac (ds}. The place of transition indicated by the ring dn is the intestinal umbilicus. The allautois (al) grows out as a small vesicle from the ventral wall of the pelvic portion of the intestinal cavity into the body-cavity of the embryo.

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Fig. 4. Longitudinal section through a Hen's egg at the beginning of the fifth day.

After the fusion of the amniotic folds, the embryo, together with the amniotic cavity (ah), is enveloped in the amniotic sac. The serous membrane (8) has been developed from the outer layer of the amniotic folds. By further separation of the middle germ-layers the extra-embryonic part of the body-cavity (Ik-) lias enlarged, and the allantois (al) has grown into it.

With the exception of a third of its surface, the } T olk has become overgrown by the outer and inner germ-layers, as far as the line ur. The vascular area has extended to the line st. The cephalic portion of the intestinal cavity has opened into the amniotic cavity by means of the newly arisen mouth (m).

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Fig. 5. Longitudinal section through a Hen's egg on the seventh day of incubation.

By the enlargement of the extra-embryonic body-cavity the serous membrane (serosa) has entirely separated from the yolk-sac, with the exception of a small area. The outer and the inner germ-layers have now grown over the yolk on all sides ; the middle germ-layer with the vascular area has extended farther downwards. The amniotic cavity, in which the embryo floats, has become much extended by the increase of the amniotic fluid. The allantois has enlarged considerably, and forms a sac, which connects with the hind gut by means of a narrow stalk (urachus). The sac extends out into the extraembryonic body-cavity between amnion, yolk-sac, and serous membrane, more particularly on the right side of the embryo.

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Fig. 6 represents a diagrammatic cross section through an embryo Fish.

The dorsal part is already far advanced in development and encloses the neural tube (N), the chorda (ch), the aorta (ao), and the primitive segments. The ventral side is greatly distended by the considerable yolk-mass (d). The latter lies in an enlargement of the intestinal canal, the intestinal yolk-sac ; this is separated from the enlarged dermal yolk-sac by means of a narrow fissure, the body-cavity (Ik).

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Fig. 7. Diagrammatic longitudinal section through a Selachian embryo.

The yolk-sac has been partly constricted off from the body of the embryo ; it still remains united to its ventral side, but only by means of a narrow stalk (sf), which consists of two tubes, one within the other, the intestinal stalk (vitelline duct) and the dermal stalk. The yolk-sac communicates with the embryonic intestinal canal by means of the vitelline duct. The point of transition is called the intestinal umbilicus (dn). The point of attachment of the dermal stalk to the belly of the embryo is the dermal umbilicus (/in). The space between dermal and intestinal umbilicus (7m and dn) serves to put the body cavity of the embryo (Ik 1 ) in communication with the body-space (Ur) between the dermal and intestinal yolk-sacs.

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Figs. 8, 9, 10, 11. Diagrammatic cross and longitudinal sections through embryo Chicks of different ages.

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Fig. 8. Half of a cross section through an i-mbryo Chick oj tiro days, after KOLLIKEE.

The embryonic body, in which the neural tube (IV), chorda (cA), primitive segment with its cavity (itsh), primitive aorta (^>),and the fundament of the primitive kidney (un) are to be seen, is marked off from the extra-embryonic region of the germ-layers by the marginal groove (gr). The body- wall begins to be developed, owing to the somatopleure having given rise to the lateral fold (sf), the ridge of which is directed toward the yolk. External to it the lateral fold of the amnion (saf) rises in an opposite direction.

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Fig. 9. Cross section of an embryo Chick at the beginning of the third day, after KOLLIKER.

The lateral folds (sf) have grown farther downward, and have completed the body-wall. The lateral folds of the amnion (saf) likewise have risen up farther toward the back of the embryo. The splanchnopleure has folded in to form the groove dr. The dotted line hn indicates the still broad dermal umbilicus, the line dn that of the intestinal umbilicus.

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Fig. 10. Cross section through the trunk of a five-days embryo Chick in the region of the umbilicus, after REMAK.

By an approximation of the lateral folds, the body-wall has been completely formed up to the region enclosed by the line 7m, in which the body-cavity still possesses an opening, and communicates with the extra-embryonic portion of the body-cavity. At the line hn, the dermal umbilicus, the body-wall bends over into the folds of the amnion (of), which have grown over the back of the embryo, and are about to fuse along their edges. At the dermal umbilicus (dn) the intestinal tube (d) passes over into the yolk-sac, which is not represented.

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Fig. 11. Diagrammatic longitudinal section through an embryo Chick.

The head is already fully differentiated from the blastoderm by the process of folding, the tail-portion is less completely separated ; the former encloses the cephalic portion of the intestinal cavity (M), which is in connection with the yolk-sac by means of the anterior intestinal portal (v.djjf). The pelvic portion of the intestinal cavity, which shows the first traces of the allantois (al), communicates backwards and above with the neural tube by means of the neurenteric canal (en), and toward the yolk-sac by means of the posterior intestinal portal (h.dpf). The head-end is already partly ensheathed by the anterior amniotic fold (vaf), whereas at the tail-end the posterior amniotic fold (Jiaf) is just beginning to be elevated.

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The Allantois

While the development of the amnion is still going on, there is formed in Reptiles and Birds an embryonic organ of no less importance, the allantois, or urinary sac. It has two different functions to perform at the same time. In the first place it serves, as its name implies, for the reception of the excretory products which are furnished during embryonic life by the kidney and primitive kidney ; and secondly, by virtue of the abundance of blood-vessels and the superficial position that it acquires, it is the most important organ of respiration.

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Fig. 128. Diagrammatic longitudinal section through the posterior end of an embryo Chick at the time of the formation of the allantois, after BUFOUR.

The section shows that the neural tube, Sp.c, is continuous at its posterior end with the hind gut, p.f.g, by means of the neurenteric canal, n.e. The latter passes through the remains of the primitive streak, pr, which is folded over toward the ventral side, ep, Outer germ-layer ; ch, chorda ; hy, entoderm (hypoblast) ; l, allantois ; me, middle germ-layer ; an, the point where the anus will arise ; m, amnion ; so, somatopleure ; .<*/>, splanchnopleure.

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The allantois takes its origin from the posterior portion of the hind gut, which is afterwards designated as the cloaca, and in the Chick the first traces of it can be recognised even at the end of the second day, at a time when the walls of the hind gut are still in the process of formation. It appears in this instance as a small csecal evagination (al} on the anterior wall of the splanchnopleure (hy) (fig. 128; Plate I., fig. 3 al).

The evagination is lined by the entoderm, and is covered externally by a growth of the splanchnic mesoderm. It enlarges rapidly into a vesicle, which grows out into the body-cavity (Plate I., fig. 4 al). At the same time the blind end enlarges, whereas the proximal part, where it passes over into the hind gut, becomes narrow and elongated into a hollow stalk, the urinary duct or urachus.

On the fourth day the urinary sac is so enlarged that it can no longer find room in the embryonic part of the body- cavity, and therefore forces itself into the extra-embryonic portion of it between the intestinal and dermal portions of the umbilical stalk (Plate I., fig. 5 al]. Here it comes into the space between the yolk-sac (ds) and amnion (/(); then it conies in contact with the inner surface of the serosa ($), and Spreads out under it for a considerable distance over the right side of the embryonic body.

In regard to the subsequent fate of the embryonic membranes in the Chick, it is to be noticed that up to the middle of incubation, i.e., up to about the eleventh day, they continue to develop in a progressive direction, but that from this time onward certain regressive processes commence, which later become more and more apparent.

In the first period (fifth to eleventh day) the following changes are effected in the yolk-sac, the amnion, the allantois, etc. The vascular area spreads out, in the manner before described, over a greater area in the wall of the yolk-sac, which still retains a considerable size. On the seventh clay it covers about two-thirds (Plate I., fig. 5), and on the tenth three-fourths of the yolk-sac. At the same time the marginal vein becomes indistinct, and the sharp separation from the non-vascular portion ceases.

The contents of the yolk-sac have become fluid by chemical changes of the yolk-mass. The serosa (S) is raised from its surface as far as the vascular area has extended, owing to the enlargement of the extra-embryonic body-cavity. At the same time the allantois (Plate I., fig. 5 al] has grown into the intermediate space. This has enlarged so much by the tenth day that it leaves uncovered only a small portion of the yolk-sac and amnion. It has lost still more of its sac-like character*; for between its outer layer, which almost everywhere is closely applied to the inner surface of the serosa, and its inner layer, adjoining the arniiion and yolk-sac, there is found only an insignificant intermediate space filled with urine.

The allantois, moreover, has by this time become a very vascular organ and is nourished by the umbilical vessels, which will engage our attention in a subsequent chapter devoted to the vascular system. The network of blood-vessels is densest in its outer laver, which spreads out at the surface of the egg ; it serves to maintain here the processes of embryonic respiration, since carbonic acid is given off from the superficially circulating blood and oxygen is taken up. The latter is acquired in part directly through the egg-shell and in part out of the air chamber (fig. 8 a.ch] situated at the blunt pole of the egg, which is in contact with a large part of the allantois.

Finally, in addition to respiration, the allantois serves for the resorption of the albumen, which becomes more and more thickened during incubation, and compressed into a lump at the pointed pole of the egg. It grows over the albumen and envelops it in a sac, the epithelial surface of which arose from the serosa, which was evaginated at the same time with the growing allantois. There are developed on the inner surface of the sac highly vascular villi, which sink into the albumen, and have been described as a placenta by DUVAL, who has called attention to these conditions.

The air chamber also has undergone modifications during incubation, and, at the same time with the acquisition of air, has increased in size by the separation of the two layers of the shell-membrane in which it is enclosed (fig. 8, p. 17).

Finally, the amnion, which at the beginning of its development is rather closely applied to the embryo, has enlarged and become a sac (Plate I., fig. 5 A] entirely filled with amniotic fluid. Its rhythmical contractions already described become most active and powerful on the eighth day, and from that time forward to the end of incubation diminish in frequency and in force.

As a result of all these processes of growth, the embryo with its appendages now demands a much larger space than at the beginning of incubation. It acquires this in the following manner. The albumen which surrounds the yolk diminishes considerably, since it disappears, especially its fluid portion, partly by evaporation to the exterior, partly also by resorption on the part of the embryo. The vitelline membrane has become ruptured by the enlargement.

In the second period, which we have reckoned from the eleventh to the twenty-first day, or to the hatching of the Chick, retrogressive metamorphoses are most prominent.

These assert themselves first of all on the yolk-sac. As the result of the vigorous sucking up of its contents it becomes more and more flaccid, so that its wall begins to lie in folds. It now becomes entirely separated from the serosa, since the extra-embryonic bodycavity has extended all around it, and thereupon it is drawn closer to the wall of the belly by the shortening of the umbilical stalk. On the nineteenth day of incubation it begins to slip into the peritoneal cavity through the dermal umbilicus, which has now become very narrow, whereby it takes on an hour-glass shape during its passage through the ventral wall. It is here employed to help in the closure of the intestinal wall.

The amnion undergoes regression, inasmuch as the fluid diminishes and almost entirely disappears, until the membrane is again closely applied to the body of the embryo. The albumen, too, is almost entirely consumed. The allantois alone continues to increase, and finally grows around so completely on the entire inner surface of the serosa that its edges come together and fuse with one another into a sac entirety enclosing the embryo and the amnion. It adheres so firmly to the serosa that a separation is no longer possible.

The urine likewise diminishes toward the end of incubation, and finally, like the amniotic fluid, has entirely disappeared. As the result of this, there are found in the allantois precipitates of uric salts, which become more and more abundant.

Amnion and allantois finally undergo complete retrogressive metamorphoses. Inasmuch as the Chick, shortly before hatching, breaks through the surrounding membranes with its bill, it begins to take in directly the air contained in the air chamber, which has become larger. A result of this is that the circulation in the allantois is retarded and finally ceases altogether. The afferent umbilical vessels disappear. Amnion and allantois die away, dry up, and then separate from the dermal umbilicus, which closes on the last day before hatching, and when the Chick leaves the egg-shell they are stripped off with it as useless remains.

Summary

1. In Reptiles and Birds the embryo during its development sinks into the underlying yolk, which has become liquefied, and becomes enveloped by folds of the extra-embryonic area of the somatopleure, the anterior, posterior, and lateral folds of the amnion (cephalic sheath, caudal sheath, lateral sheaths).
2. As the result of the folding processes two sacs arise around the embryonic body, the amnion and the serous membrane (serosa).
3. The amnion is united at the dermal umbilicus with the belly of the embryo.
4. The dermal umbilicus encloses an opening through which the embryonic and extra-embryonic portions of the body-cavity are in connection.
5. The stalk of the yolk-sac passes through the dermal umbilicus in order to attach itself to the intestine at the intestinal umbilicus.
6. The allantois is evaginated from the ventral wall of the posterior tract of the hind gut (cloaca), grows as a pedunculated sac (1) into the body-cavity, and (2) through the dermal umbilicus into the extra-embryonic part of the same, extends out from here on all sides between the amnion and serosa, and by virtue of its great vascularity functions as an organ of respiration.
7. At the end of embryonic development the constantly diminishing yolk-sac, after the consumption of the yolk, slips through the open dermal umbilicus into the body-cavity, and is employed in the closure of the intestinal umbilicus.
8. Amnion, serosa, and that part of the allantois which has grown out beyond the embryonic body, are cast off as useless structures at the dermal umbilicus, which becomes closed.

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Text-Book of the Embryology of Man and Mammals: Description of the Sexual Products | The Phenomena of the Maturation of the Egg and the Process of Fertilisation | The Process of Cleavage | General Discussion of the Principles of Development | The Development of the Two Primary Germ-Layers | The Development of the Two Middle Germ-Layers | History of the Germ-Layer Theory | Development of the Primitive Segments | Development of Connective Substance and Blood | Establishment of the External Form of the Body | The Foetal Membranes of Reptiles and Birds | The Foetal Membranes of Mammals | The Foetal Membranes of Man | The Organs of the Inner Germ-Layer - The Alimentary Tube with its Appended Organs | The Organs of the Outer Germ-Layer | The Development of the Nervous System | The Development of the Sensory Organs | The Development of the Skin and its Accessory Organs | The Organs of the Intermediate Layer or Mesenchyme | The Development of the Blood-vessel System | The Development of the Skeleton

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Cite this page: Hill, M.A. (2024, April 25) Embryology Book - Text-Book of the Embryology of Man and Mammals 11. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Text-Book_of_the_Embryology_of_Man_and_Mammals_11

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