McMurrich1914 Chapter 2
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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.
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Chapter II. The Segmentation of the Ovum and the Formation of the Germ Layers
The union of the male and female pronuclei has already been described as being accompanied by the formation of a mitotic spindle which produces a division of the ovum into two cells. This first division is succeeded at more or less regular intervals by others, until a mass of cells is produced in which a differentiation eventually appears. These divisions of the ovum constitute what is termed its segmentation.
The mammalian ovum has behind it a long line of evolution, and even at early stages in its development it exhibits peculiarities which can only be reasonably explained as an inheritance of past conditions. One of the most potent factors in modifying the character of the segmentation of the ovum is the amount of food yolk which it contains, and it seems to be certain that the immediate ancestors of the mammalia were forms whose ova contained a considerable amount of yolk, many of the peculiarities resulting from its presence being still clearly indicated in the early development of the almost yolkless mammalian ovum. To give some idea of the peculiarities which result from the presence of considerable amounts of yolk it will be well to compare the processes of segmentation and differentiation seen in ova with different amounts of it.
A little below the scale of the vertebrates proper is a form, Amphioxus, which possesses an almost yolkless ovum, presenting a simple process of development. The fertilized ovum of Amphioxus in its first division separates into two similar and equal cells, and these are made four (Fig. 17, A) by a second plane of division which cuts the previous one at right angles. A third plane at right angles to both the preceding ones brings about an eight-celled stage (Fig. 17, B), and further divisions result in the formation of a large number of cells which arrange themselves in the form of a hollow sphere which is known as a blastula (Fig. 17, E).
The minute amount of yolk which is present in the ovum of Amphioxus collects at an early stage of the segmentation at one pole of the ovum, the cells containing it being somewhat larger than those of the other pole (Fig. 17, B), and in the blastula the cells of one pole are larger and more richly laden with yolk than those of the other pole (Fig. 17, F). If, now, the segmenting ovum of an Amphibian be examined, it will be found that a very much greater amount of yolk is present and, as in Amphioxus, it is located especially at one pole of the ovum. The first three planes of segmentation have the same relative positions as in Amphioxus (Fig. 17), but one of the tiers of cells of the eight-celled stage is very much smaller than the other (Fig. 18, B). In the subsequent stages of segmentation the small cells of the upper pole divide more rapidly than the larger ones of the lower pole, the activity of the latter seeming to be retarded by the accumulation of the yolk, and the resulting blastula (Fig. 18, D) shows a very decided difference in the size of the cells of the two poles.
Fig. 17. - Stages in the Segmentation of Amphioxus. A, Four-celled stage; B, eight-celled stage; C, sixteen-celled stage; D, early blastula; B, blastula; F, section of blastula.- - (Hatschek.)
In the ova of reptiles and birds the amount of yolk stored up in the ovum is very much greater even than in the amphibia, and it is aggregated at one pole of the ovum, of which it forms the principal mass, the yolkless protoplasm appearing as a small disk upon the surface of a relatively huge mass of yolk. The inertia of this mass of nutritive material is so great that the segmentation is confined to the small yolkless disk of protoplasm and affects consequently only a portion of the entire ovum. To distinguish this form of segmentation from that which affects the entire ovum it is termed meroblastic segmentation, the other form being known as holoblastic.
Fig. 18. - Stages in the Segmentation or Amblystoma. - (Eycleshymer.)
In the ovum of a turtle or a bird the first plane of segmentation crosses the protoplasmic disk, dividing it into two practically equal halves, and the second plane forms at approximately right angles to the first one, dividing the disk into four quadrants (Fig. 19, A). The third division, like the two which precede it, is radial in position, while the fourth is circular and cuts off the inner ends of the six cells previously formed (Fig. 19, C). The disk now consists of six central smaller cells surrounded by six larger peripheral ones.
Fig. 19. - Four Stages in the Segmentation Chick. - (Coste.)
of the Blastoderm of the
Beyond this period no regularity can be discerned in the appearance of the segmentation planes; but radial and circular divisions continuing to form, the disk becomes divided into a large number of cells, those at the center being much smaller than those at the periphery. In the meantime, however, the smaller central cells have begun to divide in planes parallel to the surface of the disk, which, from being a simple plate of cells, thus becomes a discoidal cellmass.
During the segmentation of the disk it has increased materially in size, extending further and further over the surface of the yolk, into the substance of which some of the lower cells of the discoidal cell-mass have penetrated. A comparison of the diagram (Fig. 20) of the ovum of a reptile at about this stage of development with the figure of the amphibian blastula (Fig. 18, D) will indicate the similarity between the two, the large yolk-mass ( Y) of the reptile with the scattered cells which it contains corresponding to the lower pole cells of the amphibian blastula, the central cavity of which is practically suppressed in the reptile. Beyond this stage, however, the similarity becomes more obscured. The peripheral cells of the disk continue to extend over the surface of the yolk and finally completely enclose it, forming an enveloping layer which is completed at the upper pole of the egg by the discoidal cell-mass, or, as it is usually termed, the blastoderm.
Fig. 20. - Diagram Illustrating a Section of the Ovum of a Reptile at a Stage Corresponding to the Blastula of an Amphibian. bl, Blastoderm; Y, yolk-mass,
Turning now to the mammalia,* it will be found that the ovum in the great majority is almost or quite as destitute of food yolk as is the ovum of Amphioxus, with the result that the segmentation is of the total or holoblastic type. It does not, however, proceed with that regularity which marks the segmentation of Amphioxus or an amphibian, but while at first it divides into two slightly unequal cells (Fig. 21), thereafter the divisions become irregular, three-celled, four-celled, five-celled, and six-celled stages having been observed in various instances. Nor is the result of the final segmentation a hollow vesicle or blastula, but a solid mass of cells, termed a morula, is formed. This structure is not, however, comparable to the blastula of the lower forms, but corresponds to a stage of reptilian development a little later than that shown in Fig. 20, since, as will be shown directly, the cells corresponding to the blastoderm and the enveloping layer are already present. There is, then, no blastula stage in the mammalian development.
- The segmentation of the human ovum has not yet been observed; what follows is based on what occurs in the ovum of the rabbit, mole, and especially of a bat (Van Beneden).
Fig. 21. - Four Stages in the Segmentation of the Ovum of a Mouse. X, Polar globule. - (Sobolta.)
Differentiation now begins by the peripheral cells of the morula becoming less spherical in shape and later forming a layer of flattened cells, the enveloping layer, surrounding the more spherical central cells (Fig. 22, A). In the latter vacuoles now make their appearance, especially in those cells which are nearest what may be regarded as the lower pole of the ovum (Fig. 22, C), and these vacuoles, gradually increasing in size, eventually become confluent, the condition represented in Fig. 22, D, being produced. At this stage the ovum consists of an enveloping layer, enclosing a cavity which is equivalent to the yolk-mass of the reptilian ovum, the vacuolization of the inner cells of the morula representing a belated formation of yolk. On the inner surface of the enveloping layer, at what may be termed the upper pole of the ovum, is a mass of cells projecting into the yolk- cavity and forming what is known as the inner cell-mass, a structure comparable to the blastoderm of the reptile. In one respect, however, a difference obtains, the inner cell-mass being completely enclosed within the enveloping cells, which is not the case with the blastoderm of the reptile. That portion of the enveloping layer which covers the cell-mass has been termed Rauber^s covering layer, and probably owes its existence to the precocity of the formation of the enveloping layer.
It is clear, then, that an explanation of the early stages of development of the mammalian ovum is to be obtained by a comparison, not with a yolkless ovum such as that of Amphioxus, but with an ovum richly laden with yolk, such as the meroblastic ovum of a reptile or bird. In these forms the nutrition necessary for the growth of the embryo and for the complicated processes of development is provided for by the storing up of a quantity of yolk in the ovum, the embryo being thus independent of external sources for food. The same is true also of the lowest mammalia, the Monotremes, which are egg-laying forms, producing ova resembling greatly those of a reptile. When, however, in the higher mammals the nutrition of the embryo became provided for by the attachment of the embryo to the walls of the uterus of the parent so that it could be nourished directly by the parent, the storing up of yolk in the ovum was unnecessary and it became a holoblastic ovum, although many peculiarities dependent on the original meroblastic condition persisted in its development.
Fig. 22. - Later Stages in the Segmentation of the Ovum of a Bat. A, C, and D are sections, B a surface view. - (Van Beneden.)
As a rule, in the human species but one embryo develops at a time, but the occurrence of twins is by no means infrequent, and triplets and even quadruplets occasionally are developed. The occurrence of twins may be due to two causes, either to the simultaneous ripening and fertilization of two ova, either from one or from both ovaries, or to the separation of a single fertilized ovum into two independent parts during the early stages of development. That twins may be produced by this latter process has been abundantly shown by experimentation upon developing ova of lower forms, each of the two cells of an Amphioxus ovum in that stage of development, if mechanically separated, completing its development and producing an embryo of about half the normal size.
Double Monsters and the Duplication of Parts
The occasional occurrence of double monsters is explained by an imperfect separation into two parts of an originally single embryo, the extent of the separation, and probably also the stage of development at which it occurs, determining the amount of fusion of the two individuals constituting the monster. All gradations of separation occur, from almost complete separation, as seen in such cases as the Siamese twins, to forms in which the two individuals are united throughout the entire length of their bodies. The separation may also affect only a portion of the embryo, producing, for instance, double-faced or double-headed monsters or various forms of so-called parasitic monsters; and, finally, it may affect only a group of cells destined to form a special organ, producing an excess of parts, such as supernumerary digits or accessory spleens.
It has been observed in the case of double monsters that one of the two fused individuals always has the position of its various organs reversed, it being, as it were, the looking-glass image of its fellow. Cases of a similar situs inversus viscerum, as it is called, have not infrequently been observed in single individuals, and a plausible explanation of such cases regards them as one of a pair of twins formed by the division of a single embryo, the other individual having ceased to develop and either having undergone degeneration or, if the separation was an incomplete one, being included within the body of the apparently single individual. Another explanation of situs inversus has been advanced (Conklin) on the basis of what has been observed in certain invertebrates. In some species of snails situs inversus is a normal condition and it has been found that the inversion may be traced back in the development even to the earliest segmentation stages. The conclusion is thereby indicated that its primary cause may reside in an inversion of the polarity of the ovum, evidence being forthcoming in favor of the view that even in the ovum of these and other forms there is probably a distinct polar differentiation. How far this view may be applicable to the mammalian ovum is uncertain, but if it be applicable it explains the phenomenon of inversion without complicating it with the question of twin-formation.
The Formation of the Germ Layers
During the stages which have been described as belonging to the segmentation period of development there has been but little differentiation of the cells. In Amphioxus and the amphibians the cells at one pole of the blastula are larger and more yolk-laden than those at the other pole, and in the mammals an inner cell-mass can be distinguished from the enveloping cells, this latter differentiation having been anticipated in the reptiles and being a differentiation of a portion of the ovum from which alone the embryo will develop from a portion which will give rise to accessory structures. In later stages a differentiation of the inner cell-mass occurs, resulting first of all in the formation of a twolayered or diploblastic and later of a three-layered or triploblastic stage.
Fig. 23. - Two Stages in the Gastrulation of Amphioxus. - (Morgan and Hazen.)
Just as the segmentation has been shown to be profoundly modified by the amount of yolk present in the ovum and by its secondary reduction, so, too, the formation of the three primitive layers is much modified by the same cause, and to get a clear understanding of the formation of the triploblastic condition of the mammal it will be necessary to describe briefly its development in lower forms.
In Amphioxus the diploblastic condition results from the flattening of the large-celled pole of the blastula (Fig. 23, A), and finally from the invagination of this portion of the vesicle within the other portion (Fig. 23 , B) . The original single-walled blastula in this way becomes converted into a double-walled sac termed a gastrula, the outer layer of which is known as the ectoderm or epiblast and the inner layer as the endoderm or hypoblast. The cavity bounded by the endoderm is the primitive gut or archenteron, and the opening by which this communicates with the exterior is the blastopore. This last structure is at first a very wide opening, but as development proceeds it becomes smaller, and finally is a relatively small opening situated at the posterior extremity of what will be the dorsal surface of the embryo.
As the oval embryo continues to elongate in its later development the third layer or mesoderm makes its appearance. It arises as a lateral fold imp) of the dorsal surface of the endoderm (en) on each side of the middle line as indicated in the transverse section shown in Fig. 24. This fold eventually becomes completely constricted off from the endoderm and forms a hollow plate occupying the space between the ectoderm and endoderm, the cavity which it contains being the body-cavity or coelom.
In the amphibia, where the amount of yolk is very much greater than in Amphioxus, the gastrulation becomes considerably modified. On the line where the large- and small-celled portions of the blastula become continuous a crescentic groove appears and, deepening, forms an invagination (Fig. 25, gc), the roof of which is composed of relatively small yolk-containing cells while its floor is formed by the large cells of the lower pole of the blastula. The cavity of the blastula is not sufficiently large to allow of the typical invagination of all these large cells, so that they become enclosed by the rapid growth of the ectoderm cells of the upper pole of the ovum over them. Before this growth takes place the blastopore corresponds to the entire area occupied by the large yolk cells, but later, as the growth of the smaller cells gradually encloses the larger ones, it becomes smaller and is finally represented by a small opening situated at what will be the hind end of the embryo.
Fig. 24. - Transverse Section of A mphioxus Embryo with Five Mesoderms Pouches.
Ch, Notochord; d, digestive cavity; ec, ectoderm; en, endoderm; m, medullary plate; mp, mesodermic pouch. - (Halschek.)
Fig. 25. - Section through a Gastrula of Amblystoma.
dl, Dorsal lip of blastopore; gc, digestive cavity; gr, area of mesoderm formation; mes, mesoderm. - (Eycleshymer.)
Soon after the archenteron has been formed a solid plate of cells, eventually splitting into two layers, arises from its roof on each side of the median line and grows out into the space between the ectoderm and endoderm (Fig. 26, mk l and mk 2 ),. evidently corresponding to the hollow plates formed in the same situations in Amphioxus.
This is not, however, the only source of the mesoderm in the amphibia, for while the blastopore is still quite large there may be found surrounding it, between the endoderm and ectoderm, a ring of mesodermal tissue (Fig. 25, mes). As the blastopore diminishes in size and its lips come together and unite, the ring of mesoderm forms first an oval and then a band lying beneath the line of closure of the blastopore and united with both the superjacent ectoderm and the subjacent endoderm. This line of fusion of the three germ layers is known as the primitive streak. It is convenient to distinguish the mesoderm of the primitive streak from that formed from the dorsal wall of the archenteron by speaking of the former as the prostomial and the latter as the gastral mesoderm, though it must be understood that the two are continuous immediately in front of the definitive blastopore.
Fig. 26. - Section through an Embryo Amphibian (Triton) of 2% Days, showing the Formation of the Gastral Mesoderm. ok, Ectoderm; ch, chorda endoderm; dk, digestive cavity; ik, endoderm; mk 1 and mk 2 , somatic and splanchnic layers of the mesoderm. D, dorsal and V, ventral. - (Herlwig.)
In the reptilia still greater modifications are found in the method of formation of the germ layers. Before the enveloping cells have completely surrounded the yolk-mass, a crescentic groove, resembling that occurring in amphibia, appears near the posterior edge of the blastoderm, the cells of which, in front of the groove, arrange themselves in a superficial layer one cell thick, which may be regarded as the ectoderm (Fig. 27, ec), and a subjacent mass of somewhat scattered cells. Later the lowermost cells of this subjacent mass arrange themselves in a continuous layer, constituting what is termed the primary endoderm (en 1 ), while the remaining cells, aggregated especially in the region of the crescentic groove, form the prostomial mesoderm (prm). In the region enclosed by the groove a distinct delimitation of the various layers does not occur, and this region forms the primitive streak. The groove now begins to deepen, forming an invagination of secondary endoderm, the extent of this invagination being, however, very different in different species. In the gecko (Will) it pushes forward between the ectoderm and primary endoderm almost to the anterior edge of the blastoderm (Fig. 27, B), but later the cells forming its floor, together with those of the primary endoderm immediately below, undergo a degeneration, the roof cells at the tip and lateral margins of the invagination becoming continuous with the persisting portions of the primary endoderm (Figs. 27,0 and 28, B) . This layer, following the enveloping cells in their growth over the yolk-mass, gradually surrounds that structure so that it comes to lie within the archenteron. In some turtles, on the other hand, the disappearance of the floor of the invagination takes place at a very early stage of the infolding, the roof cells only persisting to grow forward to form the dorsal wall of the archenteron. This interesting abbreviation of the process occurring in the gecko indicates the mode of development which is found in the mammalia.
Fig. 27. - Longitudinal Sections through Blastoderms of the Gecko, showing â– Gastrulation. ec, Ectoderm; en, secondary endoderm; en', primary endoderm; prm, prostomial mesoderm. - (Will.)
Fig. 28. - Diagrams Illustrating the Formation of the Gastral Mesoderm in the Gecko.
ce, Chorda endoderm; ec, ectoderm; en, secondary endoderm; en 1 , primary endoderm; gm, gastral mesoderm. - (Will.)
The existence of a prostomial mesoderm in connection with the primitive streak has already been noted, and when the invagination takes place it is carried forward as a narrow band of cells on each side of the sac of secondary endoderm. After the absorption of the ventral wall of the invagination a folding or turning in of the margins of the secondary endoderrn occurs (Fig. 28), whereby its lumen becomes reduced in size and it passes off on each side into a double plate of cells which constitute the gastral mesoderm. Later these plates separate from the archenteron as in the lower forms. All the prostomial mesoderm does not, however, arise from the primitive
Fig. 29. - Sections of Ova of a Bat showing (A) the Formation of the Endoderm and (B and C) of the Amniotic Cavity. - (Van Beneden.)
In comparison with the amphibians and Amphioxus, the reptilia present a subordination of the process of invagination in the formation of the endoderm, a primary endoderm making its appearance independently of an invagination, and, in association with this subordination, there is an early appearance of the primitive streak, which, from analogy with what occurs in the amphibia, may be assumed to represent a portion of the blastopore which is closed from the very beginning.
Turning now to the mammalia, it will be found that these peculiarities become still more emphasized. The inner cell-mass of these forms corresponds to the blastoderm of the reptilian ovum, and the first differentiation which appears in it concerns the cells situated next the cavity of the vesicle, these cells differentiating to form a distinct layer which gradually extends so as to form a complete lining to the inner surface of the enveloping cells (Fig. 29, A). The layer so formed is endodermal and corresponds to the primary endoderm of the reptiles.
Before the extension of the endoderm is completed, however, cavities begin to appear in the cells constituting the remainder of the inner mass, especially in those immediately beneath Rauber's cells (Fig. 29, B), and these cavities in time coalesce to form a single large cavity bounded above by cells of the enveloping layer and below by a thick plate of cells, the embryonic disk (Fig. 29, C). The cavity so formed is the amniotic cavity, whose further history will be considered in a subsequent chapter.
It may be stated that this cavity varies greatly in its development in different mammals, being entirely absent in the rabbit at this stage of development and reaching an excessive development in such forms as the rat, mouse, and guinea-pig. The condition here described is that which occurs in the bat and the mole, and it seems probable, from what occurs in the youngest human embryos hitherto observed, that the processes in man are closely similar.
While these changes have been taking place a splitting of the enveloping layer has occurred, so that the wall of the ovum is now formed of three layers, an outer one which may be termed the trophoblast, a middle one which probably is transformed into the extra-embryonic mesoderm of later stages, though its significance is at present somewhat obscure, and an inner one which is the primary endoderm. In the bat, of whose ovum Fig. 29, C, represents a section, that portion of the middle layer which forms the roof of the amniotic cavity disappears, only the trophoblast persisting in this region, but in another form this is not the case, the roof of the cavity being composed of both the trophoblast and the middle layer.
Fig. 30. - A, Side View of Ovum of Rabbit Seven Days Old (Kdlliker); B, Embryonic Disk of a Mole (Heape); C, Embryonic Disk of a Dog's Ovum of about Fifteen Days (Bonnet) .ed, Embryonic disk; hn, Hensen's node; mg, medullary groove; ps, primitive streak; va, vascular area.
A rabbit's ovum in which there is yet no amniotic cavity and no splitting of the enveloping layer shows, when viewed from above, a relatively small dark area on the surface, which is the embryonic disk. But if it be looked at from the side (Fig. 30, A), it will be seen that the upper half of the ovum, that half in which the embryonic disk occurs, is somewhat darker than the lower half, the line of separation of the two shades corresponding with the edge of the primary endoderm which has extended so far in its growth around the inner surface of the enveloping layer. A little later a dark area appears at one end of the embryonic disk, produced by a proliferation of cells in this region and having a somewhat crescentic form. As the embryonic disk increases in size a longitudinal band makes its appearance, extending forward in the median line nearly to the center of the disk, and represents the primitive streak (Fig. 30, B), a slight groove along its median line forming what is termed the primitive groove. In slightly later stages an especially dark spot may be seen at the front end of the primitive streak and is termed Hensen's node (Fig. 30, C, hn), while still later a dark streak may be observed extending forward from this in the median line and is termed the head-process of the primitive streak.
Fig. 31. - Posterior Portion of a Longitudinal Section through the Embryonic Disk of a Mole. bl, Blastopore, ec, ectoderm; en, endoderm; prm, prostomial mesoderm. - (After Heape.)
To understand the meaning of these various dark areas recourse must be had to the study of sections. A longitudinal section through the embryonic disk of a mole ovum at the time when the crescentic area makes its appearance is shown in Fig. 31. Here there is to be seen near the hinder edge of the disk what is potentially an opening (bl) , in front of which the ectoderm (ec) and primary endoderm (en) can be clearly distinguished, while behind it no such distinction of the two layers is visible. This stage may be regarded as comparable to a stage immediately preceding the invagination stage of the reptilian ovum, and the region behind the blastopore will correspond to the reptilian primitive streak. The later forward extension of the primitive streak is due to the mode of growth of the embryonic disk. Between the stages represented in Figs. 31 and 30, B, the disk has enlarged considerably and the primitive streak has shared in its elongation. Since the blastopore of the earlier stage is situated immediately in front of the anterior extremity of the primitive streak, the point corresponding to it in the older disk is occupied by Hensen's node, this structure, therefore, representing a proliferation of cells from the region formerly occupied by the blastopore.
Fig. 32. - Transverse Section of the Embryonic Area of a Dog's Ovum at about the Stage of Development shown in Fig. 29, C.
The section passes through the head process (Chp); M, mesoderm. - (Bonnet.)
As regards the head process, it is at first a solid cord of cells which grows forward in the median line from Hensen's node, lying between the ectoderm and the primary endoderm. Later a lumen appears in the center of the cord, forming what has been termed the chorda canal, and, in some forms, including man, the canal opens to the surface at the center of Hensen's node. The cord then fuses with the subjacent primary endoderm and then opens out along the line of fusion, becoming thus transformed into a flat plate of cells continuous at either side with the primary endoderm (Fig. 32, Chp). The portion of the chorda canal which traverses Hensen's node now opens below into what will be the primitive digestive tract and is termed the neurenteric canal (Fig. t>Z, nc); it eventually closes completely, being merely a transitory structure. The similarity of the head process to the invagination which in the reptilia forms the secondary endoderm seems clear, the only essential difference being that in the mammalia the head process arises as a solid cord which subsequently becomes hollow, instead of as an actual invagination. The difference accounts for the occurrence of Hensen's node and also for the mode of formation of the neurenteric canal, and cannot be considered as of great moment since the development of what are eventually tubular structures (e. g., glands) as solid cords of cells which subsequently hollow out is of common occurrence in the mammalia. It should be stated that in some mammals apparently the most anterior portion of the roof of the archenteron is formed directly from the cells of the primary endoderm, which in this region are not replaced by the head process, but aggregate to form a compact plate of cells with which the anterior extremity of the head process unites. Such a condition would represent a further modification of the original condition.
Fig. 33. - Diagram of a Longitudinal Section through the Embryonic Disk of a Mole. am, Amnion; ce chorda endoderm; ec, ectoderm; nc, neurenteric canal; ps, primitive streak. - (Heape.)
As regards the formation of the mesoderm it is possible to recognize both the prostomial and gastral mesoderm in the mammalian ovum, though the two parts are not so clearly distinguishable as in lower forms. A mass of prostomial mesoderm is formed from the primitive streak, and when the head process grows forward it carries with it some of this tissue. But, in addition to this, a contribution to the mesoderm is also apparently furnished by the cells of the head process, in the form of lateral plates situated on each side of the middle line. These plates are at first solid (Fig. 34, gm), but their
Fig. 34. - Transverse Section through the Embryonic Disk of a Rabbit. ch, Chorda endoderm; ee, ectoderm; en, endoderm; gm, gastral mesoderm. - (After van Beneden.)
Fig. 35.- - Diagrams Illustrating the Relations of the Chick Embryo to the Primitive Streak at Different Stages of Development. - (Peebles.) cells quickly arrange themselves in two layers, between which a ccelomic space later appears.
Furthermore, as has already been pointed out, the layer of enveloping cells splits into two concentric layers, the inner of which seems to be mesodermal in its nature and forms a layer lining the interior of the trophoblast and lying between this and the primary endoderm. This layer is by no means so evident in the lower forms, but is perhaps represented in the reptilian ovum by the cells which underlie the ectoderm in the regions peripheral to the blastoderm proper (see p. 54).
It has been experimentally determined (Assheton, Peebles) that in the chick, whose embryonic disk presents many features similar to those of the mammalian ovum, the central point of the unincubated disk corresponds to the anterior end of the primitive streak and to the point situated immediately behind the heart of the later embryo and immediately in front of the first mesodermic somite (see p. 77), as shown in Fig. 35. If these results be regarded as applicable to the human embryo, then it may be supposed that in this the head region is developed from the portion of the embryonic disk situated in front of Hensen's node, while the entire trunk is a product of the region occupied by the node.
The Significance of the Germ Layers
The formation of the three germ layers is a process of fundamental importance, since it is a differentiation of the cell units of the ovum into tissues which have definite tasks to fulfil. As has been seen, the first stage in the development of the layers is the formation of the ectoderm and endoderm, or, if the physiological nature of the layers be considered, it is the differentiation of a layer, the endoderm, which has principally nutritive functions. In certain of the lower invertebrates, the class Ccelentera, the differentiation does not proceed beyond this diploblastic stage, but in all higher forms the intermediate layer is also developed, and with its appearance a further division of the functions of the organism supervenes, the ectoderm, situated upon the outside of the body, assuming the relational functions, the endoderm becoming still more exclusively nutritive, while the remaining functions, supportive, excretory, locomotor, reproductive, etc., are assumed by the mesoderm.
The manifold adaptations of development obscure in certain cases the fundamental relations of the three layers, certain portions of the mesoderm, for instance, failing to differentiate simultaneously with the rest of the layer and appearing therefore to be a portion of either the ectoderm or endoderm. But, as a rule, the layers are structural units of a higher order than the cells, and since each assumes definite physiological functions, definite structures have their origin from each.
Thus from the ectoderm there develop:
- The epidermis and its appendages, hairs, nails, epidermal glands, and the enamel of the teeth.
- The epithelium lining the mouth and the nasal cavities, as well as that lining the lower part of the rectum.
- The nervous system and the nervous elements of the senseorgans, together with the lens of the eye.
From the endoderm develop :
- The epithelium lining the digestive tract in general, together with that of the various glands associated with it, such as the liver and pancreas.
- The lining epithelium of the larynx, trachea, and lungs.
- The epithelium of the bladder and urethra (in part).
From the mesoderm there are formed:
- The various connective tissues, including bone and the teeth (except the enamel).
- The muscles, both striated and non-striated.
- The circulatory system, including the blood itself and the lymphatic system.
- The lining membrane of the serous cavities of the body.
- The kidneys and ureters.
- The internal organs of reproduction.
From this list it will be seen that the products of the mesoderm are more varied than those of either of the other layers. Among its products are organs in which in either the embryonic or adult condition the cells are arranged in a definite layer, while in other structures its cells are scattered in a matrix of non-cellular material, as, for example, in the connective tissue, bone, cartilage, and the blood and lymph. It has been proposed to distinguish these two forms of mesoderm as mesothelium and mesenchyme respectively, a distinction which is undoubtedly convenient, though probably devoid of the fundamental importance which has been attributed to it by some embryologists.
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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.
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