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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.

Textbook Contents  
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
--Mark Hill 21:14, 10 May 2011 (EST) This historic embryology textbook is at only an "embryonic" editing stage with many typographical errors and no figures.
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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Establishment of the External Form of the Body

AFTER having investigated in the preceding chapters the fundamental organs of the body of vertebrated animals, or the germ- layers, and their first important differentiations into neural tube, chorda, and primitive segments, as well as the origin of the blood and connective tissues, it will be our next undertaking to make ourselves acquainted with the development of the external form of the body, and with the development of the embryonic membranes, the latter being intimately connected with the former.

There exists an extraordinary difference in these respects between the lower and higher Vertebrates. When the embryo of an Amphioxus has passed through the first processes of development, it elongates, becomes pointed at both ends, and already possesses in the main the worm-like or fish-like form of the adult animal. But the higher we ascend in the series of Vertebrates, the more are the embryos, when they attain the stage of development corresponding to the Amphioxus embryo, unlike the adult animals : at this stage they assume very singular and strange forms, inasmuch as they become surrounded by peculiar envelopes and are provided with various appendages, which subsequently disappear.

The difference is referable, first of all, to the more or less extensive accumulation of nutritive yolk, the significance of which for the nascent organism is twofold.

From a physiological point of view, the nutritive yolk is a rich source of energy which alone makes it possible for the embryological processes to take place in uninterrupted sequence, until at length an organism, with an already relatively high organisation, begins its independent existence.

From a morphological point of view, on the other hand, the yolk plays the role of ballast, which exerts a restrictive and modifying influence on the direct and free development of those organs which are entrusted with the reception and elaboration of it. Even at the very beginning of development we could see how the cleavage-process and the formation of the germ-layers were retarded, altered, and to a certain extent even suppressed by the presence of yolk. In what follows we shall again have occasion to point out the same thing, how, owing to the presence of yolk, the normal formation of the intestinal canal and of the body can be attained only gradually and by a circuitous process.

In the second place, the great difference which the embryos of Vertebrates present is produced by the medium in which the eggs undergo development. Eggs which, like those of water-inhabiting Vertebrates, are deposited in the water, are developed in a more simple and direct manner than those which, provided with a firm shell, are laid upon the land, or than those which are enclosed in the womb up to the time of the birth of the embryos.

In the two latter cases the growing organism attains its goal only by very indirect ways. At the same time with the permanent organs there are also developed others which have no significance for the post-embryonic life, but which serve during the egg-stage of existence either for the protection of the soft, delicate, and easily injured body, or for respiration, or for nutrition. These either undergo regressive metamorphosis at the end of embryonic life, or are cast oft' at birth as useless and unimportant structures. But inasmuch as they are developed out of the germ-layers, they are also properly to be regarded as belonging immediately to the nascent organism as being its embryonic organs, and as such they too are to be treated in morphological descriptions.

The extensive material which has to be mastered in this connection I shall present grouped into tivo parts.

In the first part we shall inquire how the embryo overcomes the obstacle which it encounters in the presence of the yolk and acquires its ultimate form.

In the second and likewise more extensive part we must concern ourselves more minutely with the embryonic enveloping structures and appended organs, which subserve various purposes.


The collection of yolk-material disturbs the course of development least in the case of the Amphibia. The latter therefore stand, as it were, midway between Amphioxus with direct development and the remaining Vertebrates, and constitute a transition between them. In the Amphibia the yolk shares in the process of cleavage ; after the close of this process it is found accumulated for the most part in the large yolk-cells which form the floor of the blastula (fig. 45) ; at the time of the differentiation into germFig. 118. Diagrammatic longitudinal section through layers it is taken up into the the embryo of a Frog, after GOETTE, from BALPOUR. cffilenteron. which it almost nc, Neural tube ; a-, communication of the same with blastopore and ccelenteron (at) ; ?/Jt, yolk-cells ; m, Completely fills (fig. 47); after middle germ-layer. For the sake of simplicity the ^ formafcion f the bod outer germ-layer is represented as it composed 01 > a single layer of cells. sacs the large yolk-cells lie in a similar manner in the ventral wall of the intestine proper (fig. 118 yti). Here they are in part dissolved and employed for the growth of the remaining parts of the body, in part they share directly in the formation of the epithelium of the ventral wall of the intestine.

In consequence of the presence of the great accumulation of yolkcells, the Amphibian embryo acquires a shapeless condition at a time when the Amphioxus larva has already become elongated and fishlike. The body, which is spherical during gastrulation, later becomes egg-shaped, owing to its elongation. Thereupon the head-end and the tail-end begin to be established at the two poles as small elevations (figs. 118 and 80). The middle or trunk-part lying between the latter becomes somewhat incurved along its dorsal region, in which neural tube, chorda, and primitive segments are developed, so that the cephalic and caudal elevations become joined by means of a concave line. The ventral side of the trunk-region, on the contrary, is greatly swollen and bulges out ventrally and laterally like a hernia, since it is filled with yolk-cells. This swelling is therefore called the yolk-sac.

In the further progress of development the embryo continually acquires a more fish-like shape. The anterior and the posterior ends of the body, especially the latter, increase greatly in length, and the middle of the trunk becomes thinner, for with the consumption of the yolk-material the yolk-sac becomes smaller and finally disappears altogether, its walls being incorporated into the ventral wall of the intestine and that of the body.

The interferences in the normal course of development become greater in the same ratio as the yolk increases in amount, as it does in the case of the meroblastic eggs of Fishes, Reptiles, and Birds. With the latter the yolk is no longer broken up into a mass of yolk-cells, as in the case of the Amphibia ; it participates in the process of cleavage, but only to a slight extent, inasmuch as nuclei make their way into the layer of yolk which is adjacent to the germ, and, surrounded by protoplasm, continue to increase in number by division. The gastrula-form is altered until it becomes unrecognisable; only a small part of its dorsal surface consists of cells, which are arranged into the two primary germ-layers, whereas the whole ventral side, where in the Amphibia the yolk-cells are found, is an unsegmented yolk-mass.

Thus we acquire in the case of the Vertebrates mentioned a peculiar condition ; the embryo, if we regard the yolk as not belonging to the body, appears to be developed from layers that are spread out flat instead of from a cup-like structure (Plate I., fig. 1, page 213). Moreover we see even a greater distinction effected between the dorsal and ventral surfaces of the egg during development than was the case with the Amphibians. The fundaments of all important organs, the nervous system, the chorda, the primitive segments (Plate I., figs. 2, 8), are at first produced exclusively on the former, whereas on the ventral side few and unimportant changes only are to be observed. These consist principally in the extension of the germ-layers, which spread out farther ventrally, grow over the yolkmass (Plate I., figs. 2-5), and form around it a closed sac consisting of several layers. This circumcrescence of the unsegmented yolk by the germ-layers is accomplished, on the whole, very slowly, the more voluminous the accumulated yolk-material, the more time it requires : thus, for example, in the case of Birds it is completed at a very late stage of development, when the embryo has already attained a high state of perfection (Plate I., fig. 5).

In the case of nieroblastic eggs, the part of the germ-layers on which the first fundaments of the organs (neural tube, chorda, primitive segments, etc.) appear has been distinguished as the embryonic area from the remaining part, or the extra-embryonic area. The distinction is both fitting and necessary ; but the names might have been more appropriate than " embryonic and extra-embryonic," since obviously everything that arises from the egg-cell, and consequently even that Eni which originates in the extra-embryonic area, must be reckoned as belonging to the embryo. The differentiation into two areas persists in the course of further development, and becomes expressed still more sharply (fig. 119). The embryonic area, by means of the folding of its flattened layers into tubes, alone forms the elongated, fish-like body which all Vertebrates at first exhibit ; the extra-embryonic area, on the contrary, becomes a sac filled with yolk (ds), which, like an enormous hernia, is united to the embryo (Em) by means of a stalk (st) attached to its belly, sometimes even while the embryo is still remarkably sma

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Fig. 119. Advanced embryo of a Shark (Pristiurus), after BALFOUR.

Em, Embryo ; ds, yolk-sac ; st, stalk of the yolk-sac ; av, arteria vitellina ; vv, vena vitellina.

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ll.

We must now explain more minutely the details of the processes of development which take place in this connection : first the metamorphosis of the flattened embryonic area into the fish-like embryonal body, and secondly the formation of the yolk-sac.

In the presentation Ave shall adhere chiefly to the Hen's egg, but for the time being we shall leave out of consideration the formation of the embryonic membranes.

The body of the Chick is developed by a folding of the flattened layers, and by the constricting off of the tubular structures thus formed from the area pellucida. The beginning of the process of folding is recognisable upon the surface of the blastoderm by means of certain furrows, the marginal grooves (Grenzrinnen) of His. These appear earlier in the anterior than in the posterior region of the embryonic fundament, in correspondence with the law previously enunciated, according to which the anterior end of the body anticipates in development the posterior end.

At first that part of the embryonic fundament which is destined to become the head is marked off by means of a crescentic groove (fig. 120). In the case of the Chick this is indicated during the first day of incubation, at a time when the first trace of the nervous system becomes visible. It lies immediately in front of the curved anterior end of the medullary ridges, with its concavity directed backward.

At a later stage the embryonic area is marked off laterally. In the Case of the embryo Seen from Fig. 120. -Surface-view of the area pellucida of . . . a blastoderm of 18 hours, after BALFOUR.

the SUMace m fag. 121, m Which ]u fl , )llf nf the primitive groove (//) lies the mt-diillaiy furrow (//if), with the medullary ridges (A). The*e diverge belaud and fade out ou either side iu front of the primitive groove ; anteriorly, ou the contrary, they are continuous with each other, and form an arch behind a curved line, which represents the anterior marginal groove. The second ciirved line, lying in front of and concentric with the lirst, is the beginning of the amniotic fold.


the neural tube is already partly closed and segmented into three brain-vesicles, and in which six pairs of primitive segments are laid down, there may be recognised at some distance from these primitive segments two dark streaks, the two lateral marginal grooves. They become less distinct in passing from before backward, and wholly disappear at the end of the primitive groove.

Finally, the tail-end of the embryo is marked off by the posterior marginal groove, which like the anterior is crescentic, but has its concavity directed toward the head.

In this manner a small part of the germ-layers, which alone is required for the construction of the permanent body, is separated by a continuous marginal furrow from the much more extensive extraembryonic area, which serves for the formation of evanescent organs like the yolk-sac and the embryonic membranes.

hb , The marginal grooves are formed by the infolding of the outer germ-layer hi" and the parietal middle layer, which are together called the somatopleure, and in such a manner that the A ridge of the original small fold is directed downward toward the yolk (Plate I., fig. 8 sf). The space enclosed by the two folded layers is the marginal groove (</r). As we have distinguished on the latter several regions, which are developed at different times, so must we here distinguish the corresponding folds, pr and we consequently speak of a headfold, a tailfold, and the two lateral folds.

The keadfold ' appears, first of all, even on the first, but more distinctly on the second, day of incubation. By means of it the head-end of the embryonal fundament is formed and separated from the extra -embryonic part of the germ-layers. At the moment of its origin it is turned directly downward toward the yolk; but the more it enlarges, whereby the anterior marginal

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Fig. 121. Blastoderm of the Chick, incubated 33 hours, after DUVAL.

< >ue sees the pellucid area, lif t surrounded by a portion of the opaque area, <(/'. The fundament of the nervous system is closed anteriorly and segmented into three brain-resides, kb l , hb", hb ; belaud, the medullary fold mf is stil] open. On either side of it lie six primitive segments, us. The posterior end of the fundament of the embryo is occupied by the primitive streak with the primitive groove, pr. groove is deepened into a pit, the more its ridge is turned backwards.

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Two diagrammatic longitudinal sections, one of which is shown in fig. 122, the other on Plate I., fig. 11, may serve to illustrate this process.

In fig. 122 there is shown, projecting above the otherwise smooth flat surface of the germ-layers, a small protuberance, which encloses the anterior end of the neural tube (^V.6') and the simultaneously forming intestinal tube (Z)), and which has arisen by the formation of the fold F.So. The upper sheet of the fold, by directing itself backwards, furnishes the ventral wall of the cephalic elevation ; the lower sheet forms the floor of the marginal groove.


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Fig. 122. Diagrammatic longitudinal section through the axis of an embryo Bird, after BALFOUR. The section represents the condition when the head-fold has begun, but the tail-fold is still wanting. F.So, Head-fold of the soruatopleure ; F.Sp, head- fold of the splauchnopleure, forming at Sp the lower wall of the front end of the mesenteron ; D, cavity of the fore gut ; pp, pleuroperitoneal cavity ; Am, fundament of the anterior fold of the amnion ; N.C, neural tube ; Ch, chorda ; A, B, C, outer, middle, inner germ-layer, everywhere distinguished by different shading; HI, heart.

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In the second figure, in which there is represented a diagrammatic longitudinal section through an older embryo, the head-fold (kf 1 ) has extended still farther backward. The head has thereby become longer, since its under surface has increased in consequence of the advance in the process of folding.

Whoever desires to make this process, which is very important for the comprehension of the construction of animal forms, clearer and more intelligible, may do so with the help of an easily constructed model. Let him stretch out his left hand on a table, and spread flat over the back of it a cloth, which is to represent the blastoderm ; then let him fold in the cloth with his right hand by tucking it a little way under the points of his left fingers. The artificially produced fold corresponds to the head-fold previously described. The points of the lingers, which by the tucking under of the cloth have received a covering on their lower sides, and which project above the otherwise flattened cloth, are comparable to the cephalic elevation. In addition we can represent the backward growth of the head-fold by tucking the cloth still farther under the left fingers toward the wrist.

The hinder end of the embryo develops in the same manner as the front end, only somewhat later (compare fig. 11, Plate I.). Corresponding to the posterior marginal groove (#r), the tail-fold is so formed that its ridge is directed forward and that it grows toward the head-fold.

Where in surface-views of the blastoderm the lateral marginal grooves are to be seen (fig. 121), one recognises on cross sections the lateral folds (Plate I., fig. 8 sf). They grow at first directly from above downwards, thus producing the lateral walls of the trunk. Afterwards their margins bend somewhat toward the median plane (Plate I., fig. 9 sf), thereby approaching each other, and in this way gradually draw together to form a tube (Plate I., fig. 10). By their infolding the trunk acquires its ventral wall.

In order to avoid misconceptions, let it be further remarked that only at the beginning of their formation are head-, tail-, and lateral folds somewhat separated from one another, but that when they are more developed they are merged into one another, and thus are only parts of a single fold, which encloses the fundament of the embryo on all sides.

As the separate parts of this fold increase, they grow with their bent margins from in front and from behind, from right and from left, toward one another, and finally come near together in a small territory, which corresponds approximately with the middle of the surface of the embryo's belly, and is designated on the figure of the cross section through this region (Plate I., fig. 10) by a ring-like line (Jin}. Thus a small tubular body is formed (Plate I., fig. 3), which lies upon the extra-embryonic area of the blastoderm and is united to it by means of a hollow stalk (7m). The stalk marks the place where the margins of the folds, growing toward one another from all sides, have met, but a complete constricting off of the embryonic territory from the extra-embryonic does not take place.

We can also represent these conditions, if, in the previously mentioned model, we in addition fold in the cloth that covers the tips of the fingers along the sides of the hand and the wrist, and then carry the circular fold thus artificially formed still farther under, even to the middle of the palm. Then the cloth forms around the hand a tubular sheath, which is continuous at one place by means of a connecting cord with the flattened remaining portion of the cloth.

A process similar to the externally visible one just described, by which the lateral and ventral walls of the body are produced from the sheet-like fundaments, takes place at the same time within the embryo in the splanchnopleure. There are developed from it, as from the somatopleure, an anterior, a posterior, and two lateral intestinal folds.

First, at the time when the head is differentiated (fig. 122), the part of the splanchnopleure corresponding to it (F.tSj).} is folded together into a tube, the so-called cavity of the fore gut or liead-gut (D).

The same process repeats itself on the third day of incubation at the posterior end of the embryonal fundament, where, upon the appearance of the caudal part (Plate I., fig. 11), there is formed within it and out of the splanchnopleure the cavity of the hind gut.

Both parts of the intestine at first terminate with blind ends directed toward the outer surface of the body. At the head-end the mouth-opening is still wanting, at the posterior end the anus. When, however, one raises the blastoderm with the nascent embryo from the yolk, Mid examines it from the under side, the anterior and posterior portions of the intestinal canal exhibit openings (vdpf and Julpf). through which one can look from the yolk-side into the blind-ending cavities. One of these is called the anterior, the other the posterior, intestinal portal or intestinal entrance (Plate I., fig. 11 vdpf &nd lidpf}.

Between the two portals the middle region of the intestinal canal remains for a long time as a leaf-like fundament. Then by its becoming somewhat bent downwards (Plate I., figs. 9 and 2) there raises under the chorda dorsalis an intestinal groove (dr), which lies between fore and hind gut. Owing to the further increase of the lateral intestinal folds (df), the groove becomes deeper and deeper, and finally, by the approximation of the edges of the folds from in front, from behind, and from both sides, becomes closed into a tube in the same manner as the wall of the body.

At only one small place, which is indicated by the ring-like line dn hi Plate I., figs. 3 and 10, the folding and constricting-off process is not completed, and here the intestinal tube too remains continuous, by means of a hollow stalk, with the extra -embryonic part of the splanchnopleure, which encloses the yolk.

The part of the germ-layers which is not employed in the formation of the embryo furnishes in the case of the Reptiles and Birds the yolk-sac and certain embryonic membranes. I shall speak of the development of these in the next chapter.

The fate of the extra- embryonic area of the blastoderm in Fishes is more sinfple, since there is formed from it only a sac for the reception of the yolk.

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Fig. 123 exhibits the embryo (Em) of a Selachian, which has arisen by the infolding of a small area of the germ-layers in the manner described for Em the Chick. All the remaining part of the egg has become a great yolk-sac (ds), which is united with the middle of the belly by means of a long stalk.

The Teleosts (Plate I., fig. 6) show us transitions from this Fig. 123. Advanced embryo of a Shark (Pristiurus), after condition to O116 in BALFOUR. ] ' 1 + 1 1L En, Embryo ; ds, yolk-sac ; st, stalk of the yolk-sac ; ac, arteria wnicn tne yolK-SaC, \itellina; vv t vena vitellina. as ill Amphibians, is not separated by a stalk from the rnesenteron, but represents only a capacious enlargement of the latter and of the belly-wall.

Let us now examine more carefully the structure of the yolk-sac. As has been remarked already, all four of the germ-layers spread themselves out one after another around the unsegnaented yolk-mass of meroblastic eggs (Plate I., figs. 6 and 7). As in the embryonal body the two middle germ-layers separate from each other and allow the body-cavity to appear between them, so, too, at a later stage the same process occurs in the extra-embryonic area. Throughout the region of the middle germ-layer there is formed a narrow fissure, for which the name " extra-embryonic body-cavity," or blastospheric culom (cavity of the blastoderm, KOLLIKER), would be most suitable. It separates the envelope of the yolk into two layers, of which the inner is the immediate continuation of the. intestinal wall (splanchiiopleure), the outer, on the contrary, that of the bodywall (somatopleure). Therefore, to be exact, we have before us a double sac formed around the yolk, which we can distinguish as intestinal yolk-sac and dermal yolk-sac. The former is simply a hernia-like evagination of the intestinal canal, and, like it, is composed of three layers :

(1) The intestine-glandular layer (ik), the entoblast or secondary entoderm, which encloses the yolk ;

(2) The visceral middle layer, or the pleuroperitoneal epithelium (mk 2 ) ; and

(3) The intermediate layer (Zwischenblatt), in which have been developed the vitelline blood-vessels, which at the beginning of the circulation of the blood have to conduct the liquefied nutritive material from the yolk-sac to the places of embryonic growth.

The dermal yolk-sac is, as a continuation of the body-wall, likewise composed of three layers the epidermis (&&), the parietal middle layer (mk l ), and the connective-tissue intermediate substance (Zwischensubstanz).

It has already been stated that the constricting-off of the yolk-sac from the embryonal body is quite variable in extent, and can go so far that the connection between the two is kept up only by means of a narrow stalk. A more careful examination shows that in the latter case the stalk itself is composed of two narrow tubes one within the other (Plate I., fig. 7), of which the outer unites the dermal yolk-sac (hs) to the ventral wall of the body, and the inner the intestinal yolk-sac to the intestinal canal. The former is called the dermal stalk, the latter the intestinal stalk (dii) or vitelline duct, ductus vitello-intestinalis. The place of attachment of the dermal stalk in the middle of the ventral surface of the embryo is called the dermal navel (Jin) ; the corresponding place of attachment of the intestinal stalk to the wall of the intestine the intestinal navel (dn). The embryonic body-cavity opens out between the two, and is continuous with the fissure between dermal and intestinal yolk-sac with the " extra-embryonic body-cavity ' : or the blastospheric coelom (Ui 2 ).

The ultimate fate of the yolk-sac in the Fishes is the same as in the Amphibia. It is still employed, even in the extreme case of the Selachians, for the formation of the wall of the intestine and that of the body. The more its contents are liquefied and absorbed, the more the yolk-sac shrivels. When the intestinal yolk-sac has become very small, it is drawn into the body-cavity and finally serves to close the intestinal navel, just as the dermal yolk-sac upon its disappearance closes up the dermal navel. With the lower Vertebrates a shedding of the embryonic parts has not yet come into existence. The next chapter will explain what becomes of the yolk-sac in the case of Reptiles and Birds.

Summary

  1. In the case of Vertebrates whose eggs contain little yolk, the embryo after the development of the germ-layers takes on an elongated, fish-like form.
  2. In eggs with abundant yolk the body of the vertebra ted animal is produced by only a small region of the germ -layers (the embryonic fundament) ; the far greater extra-embryonic area is employed for the formation of a yolk-sac and of embryonic membranes (the latter only in Reptiles and Birds).
  3. The separate layers of the embryonic fundament constrict themselves off from the extra-embryonic territory, and at the same time become folded into tubes the somatopleure into the tubular bodywall, the splanchnopleure into the intestinal tube (head-fold, tail-fold, lateral folds, intestinal groove, intestinal fold).
  4. The extra -embryonic territory of the germ-layers remains in continuity with the two tubes by means of a stalk-like connection.
  5. In Fishes the extra-embryonic territory of the germ-layers becomes the yolk-sac, which is composed of two sacs, the intestinal and the dermal yolk-sacs, separated from each other by a prolongation of the embryonal body-cavity.
  6. The place where the dermal yolk-sac is attached to the belly wall of the embryo by a stalk-like prolongation is called the dermal navel or umbilicus ; the corresponding place of attachment of the intestinal yolk-sac to the middle of the intestinal canal is the intestinal navel or umbilicus.
  7. In Fishes the yolk-sac after resorption of the yolk-material, accompanied by the phenomena of shrivelling, is employed for the closure of the intestinal and dermal navels.
  8. In Reptiles and Birds the extra-embryonic region furnishes, in addition to the yolk-sac, several other embryonic membranes, which complicate the development.

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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