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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.
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THE SEGMENTATION OF THE OVUM AND THE FORMATION OF THE GERM LAYERS.
Segmentation. - 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
Fig. 23. - Two Stages in the Gastrulation of Amphioxus. - (Morgan and Hazen.) 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.
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
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.) 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.
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: i. The epidermis and its appendages, hairs, nails, epidermal glands, and the enamel of the teeth.
2. The epithelium lining the mouth and the nasal cavities, as well as that lining the lower part of the rectum.
3. The nervous system and the nervous elements of the senseorgans, together with the lens of the eye.
From the endoderm develop : 1. The epithelium lining the digestive tract in general, together with that of the various glands associated with it, such as the liver and pancreas.
2. The lining epithelium of the larynx, trachea, and lungs.
3. The epithelium of the bladder and urethra (in part). From the mesoderm there are formed: 1. The various connective tissues, including bone and the teeth (except the enamel).
2. The muscles, both striated and non-striated.
3. The circulatory system, including the blood itself and the lymphatic system.
4. The lining membrane of the serous cavities of the body.
5. The kidneys and ureters.
6. 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.
R. Assheton: "The Reinvestigation into the Early Stages of the Development of the Rabbit," Quarterly Journ. of Microsc. Science, xxxvn, 1894. R. Assheton: "The Development of the Pig During the First Ten Days," Quarterly Journ. of Microsc. Science, xli, 1898. R. Assheton: "The Segmentation of the Ovum of the- Sheep, with Observations on the Hypothesis of a Hypoblastic Origin for the Trophoblast," Quarterly Journ.
of Microsc. Science, xli, 1898. E. van Beneden: "Recherches sur les premiers stades du developpement du Murin (Vespertilio murinus)," Anatom. Anzeiger, xvi, 1899. R. Bonnet: "Beitrage zur Embryologie der Wiederkauer gewonnen am Schafei," Archivfiir Anat. und Physiol., Anat. Abth., 1884 and 1889. R. Bonnet: "Beitrage zur Embryologie des Hundes," Anat. Hefte, ix, 1897. G. Born: "Erste Entwickelungsvorgange," Ergebnisse der Anat. und Entwicklungs gesch., 1, 1892.
E. G. Conklin: "The Cause of Inverse Symmetry," Anatom. Anzeiger, xxm, 1903.
A. C. Eycleshymer: "The Early Development of Amblystoma with Observations on Some Other Vertebrates," Journ. of Morphol., x, 1895.
B. Hatschek: "Studien uber Entwicklung des Amphioxus," Arbeiten aus dem zoolog.
Ins tit. zu Wien, rv, 1881. W. Heape: "The Development of the Mole (Talpa europaea)," Quarterly Journ. of Microsc. Science, xxm, 1883. A. A. W. Hubrecht: "Studies on Mammalian Embryology II: The Development of the Germinal Layers of Sorex vulgaris," Quarterly Journ. of Microsc. Science, xxxi, 1890.
F. Keibel: "Studien zur Entwicklungsgeschichte des Schweines," Morpholog.
Arbeiten, in, 1893. F. Keibel: "Die Gastrulation und die Keimblattbildung der Wirbeltiere," Ergebnisse der Anat. und Entwicklungsgesch., x, 1901. M. KunsemVJller: "Die Eifurchung des Igels (Erinaceus europasus L.)," Zeitschr.
fiir wissensch. Zool., lxxxv, 1906. K. Mitsukuri and C. Ishikawa: "On the Formation of the Germinal Layers in Chelonia," Quarterly Journ. of Microsc. Science, xxvn, 1887. F. Peebles: "The Location of the Chick embryo upon the Blastoderm," Journ. of Exper. Zool., 1, 1904. E. Selenka: " Studien uber Entwickelungsgeschichte der Thiere," 4tes Heft, 1886-87; 5tes Heft, 1891-92. J. Sobotta: "DieBefruchtungundFurchungdesEies der Maus," Archivfiir mikrosk.
Anat., xlv, 1895.
J. Sobotta: " Die Furchung des Wirbelthiereies," Ergebnisse der Anal, unci Entwicke lungsgeschichte, vi, 1897. J. Sobotta: "Neuere Auschauungen iiber die Entstehung der Doppel (miss) bild ungen, mit besonderer Beriicksichtigung der menschlichen Zwillingsgeburten," Wiirzburger Abhandl., I, 1901. H. H. Wilder: "Duplicate Twins and Double Monsters," Amer. Jour, of Anal., in, 1904. L. Will: "Beitrage zur Entwicklungsgeschichte der Reptilien," Zoolog. Jahrbilcher Abth.fur Anal., vi, 1893.
The Development of the Optic Nerve
The observations on the development of the retina have shown very clearly that the great majority of the fibers of the optic nerve are axis-cylinders of the ganglion cells of the retina and grow from these cells along the optic stalk toward the brain. Their embryonic history has been traced most thoroughly in rat embryos (Robinson), and what follows is based upon what has been observed in that animal.
The optic stalk, being an outgrowth from the brain, is at first a hollow structure, its cavity communicating with that of the third ventricle at one end and with that of the optic bulb at the other. When the chorioid fissure is developed, it extends, as has already been described, for some distance along the posterior surface of the stalk and has lying in it a portion of the hyaloid artery. Later, when the lips of the fissure fuse, the artery becomes enclosed within the stalk to form the arteria centralis retina of the adult (Fig. 276). By the formation of the fissure the original cavity of the distal portion of the stalk becomes obliterated, and at the same time the ventral and posterior walls of the stalk are brought into continuity with the retinal layer of the optic cup, and so opportunity is given for the passage of the axis-cylinders of the ganglion cells along those walls (Fig. 274). At an early stage a section of the proximal portion of the optic stalk (Fig. 275, A) shows the central cavity surrounded by a number of nuclei representing the mantle layer, and surrounding these a non-nucleated layer, resembling the marginal velum and continuous distally with the similar layer of the retina. When the ganglion cells of the latter begin to send out their axis-cylinder processes, these pass into the retinal marginal velum and converge in this layer toward the bottom of the chorioidal fissure, so reaching the ventral wall of the optic stalk, in the velum of which they may be distinguished in rat embryos of 4 mm., and still more clearly in those of 9 mm. (Fig. 275, A). Later, as the fibers become more numerous, they gradually invade the lateral and finally the dorsal
Fig. 2 74. - Diagrammatic Longitudinal Section of the Optic Cup and Stalk passing through the chorioid fisSURE.
Ah, Hyaloid artery; L, lens; On, fibers of the optic nerve; Os, optic stalk; PI, pigment layer, and R, retinal layer of the retina.
THE OPTIC NERVE
walls of the stalk, and, at the same time the mantle cells of the stalk become more scattered and assume the form of connective-tissue (neuroglia) cells, while the original cavity of the stalk is gradually obliterated (Fig. 275, B). Finally, the stalk becomes a solid mass of nerve-fibers, among which the altered mantle cells are scattered.
Fig. 275. - Transverse Sections through the Proximal Part of the Optic Stalk of Rat Embryos of (A) 9 mm. and (5) 11 mm. - (Robinson.)
From what has been stated above it will be seen that the sensory cells of the eye belong to a somewhat different category from those of the other sense-organs. Embryologically they are a specialized portion of the mantle layer of the medullary canal, whereas in the other organs they are peripheral structures either representing or being associated with representatives of posterior root ganglion cells. Viewed from this standpoint, and taking into consideration the fact that the sensory portion of the retina is formed from the invaginated part of the optic bulb, some light is thrown upon the inverted arrangement of the retinal elements, the rods and cones being directed away from the source of light. The normal relations of the mantle layer and marginal velum are retained in the retina, and the latter serving as a conducting layer for the axis-cylinders of the mantle layer (ganglion) cells, the layer of nerve-fibers becomes interposed between the source of light and the sensory cells. Furthermore, it may be pointed out that if the differentiation of the retina be imagined to take place before the closure of the medullary canal - a condition which is indicated in some of the lower vertebrates - there would be then no inversion of the elements, this peculiarity being due to the conversion of the medullary plate into a tube, and more especially to the fact that the retina develops from the outer wall of the optic cup. In certain reptiles in which an eye is developed in connection with the epiphysial outgrowths of the diencephalon, the retinal portion of this pineal eye is formed from the inner layer of the bulb, and in this case there is no inversion of the elements.
A justification of the exclusion of the optic nerve from the category which includes the other cranial nerves has now been presented. For if the retina be regarded as a portion of the central nervous system, it is clear that the nerve is not a nerve at all in the strict sense of that word, but is a tract, confined throughout its entire extent within the central nervous system and comparable to such groups of fibers as the direct cerebellar or fillet tracts of that system.
The Development of the Vitreous Humor. - It has already been pointed out (p. 448) that a blood-vessel, the hyaloid artery, accompanied by some mesodermal tissue makes its way into the cavity
Fig. 276. - Reconstruction of a Portion of the Eye of an Embryo of 13.8 mm. ah, Hyaloid artery; ch, chorioid coat; /, lens; r, retina. - (His.) of the optic cup through the chorioid fissure. On the closure of the fissure the artery becomes enclosed within the optic stalk and appears to penetrate the retina, upon the surface of which its branches ramify. In the embryo the artery does not, however, terminate in these branches as it does in the adult, but is continued on through the cavity of the optic cup (Fig. 276) to reach the lens, around which it sends branches to form the tunica vasculosa lentis.
According to some authors, the formation of the vitreous humor is closely associated with the development of this artery, the humor being merely a transudate from it, while others have maintained that it is a derivative of the mesoderm which accompanies the vessel, and is therefore to be regarded as a peculiar gelatinous form of connective tissue. More recently, however, renewed observations by several authors have resulted in the deposition of the mesoderm from the chief role in the formation of the vitreous and the substitution in it of the retina. At an early stage of development delicate protoplasmic processes may be seen projecting from the surface of the retinal layer into the cavity of the optic cup, these processes probably arising from those cells which will later form the Muller's
Fig. 277. - Transverse Section through the Ciliary Region of a Chick Embryo of Sixteen Days. ac, Anterior chamber of the eye; cj, conjunctiva; co, cornea; i, iris; I, lens; mc, ciliary muscle; rl, retinal layer of optic cup; sf, spaces of Fontana; si, suspensory ligament of the lens; v, vitreous humor. - (Angelucci.) (neuroglia) fibers of the retina. As development proceeds they increase in length, forming a dense and very fine fibrillar reticulum traversing the space between the lens and the retina and constituting the primary vitreous humor. The formation of the fibers is especially active in the ciliary portion of the retina and it is probable that it is from some of the fibers developing in this region that the suspensory ligament of the lens (zonula Zinnii) (Fig. 277, si) is formed spaces which occur between the fibers of the ligament enlarging to produce a cavity traversed by scattered fibers and known as the canal of Petit.
A participation of similar protoplasmic prolongations from the cells of the lens in the formation of the vitreous humor has been maintained (von Lenhossek) and as strenuously denied. But it is generally admitted that at the time when the hyaloid artery penetrates the vitreous to form the tunica vasculosa lentis it carries with it certain mesodermal elements, whose fate is at present uncertain. It has been held that they take part in the formation of the definitive vitreous, which, according to this view, is of mixed origin, being partly ectodermal and partly mesodermal (Van P6e), and, on the contrary, it has been maintained that they eventually undergo complete degeneration, the vitreous being of purely ectodermal origin (von Kolliker).
The degeneration of the mesodermal elements which the latter view supposes is associated with the degeneration of the hyaloid artery. This begins in human embryos in the third month and is completed during the ninth month, the only trace after birth of the existence of the vessel being a more fluid consistency of the axis of the vitreous humor, this more fluid portion representing the space originally occupied by the artery and forming what is termed the hyaloid canal (canal ofCloquet).
The Development of the Outer Coat of the Eye, of the Cornea, and of the Anterior Chamber. - Soon after the formation of the optic bulb a condensation of the mesoderm cells around it occurs, forming a capsule. Over the medial portions of the optic cup the further differentiation of this capsule is comparatively simple, resulting in the formation of two layers, an inner vascular and an outer denser and fibrous, the former becoming the chorioid coat of the adult eye and the latter the sclera.
More laterally, however, the processes are more complicated. After the lens has separated from the surface ectoderm a thin layer of mesoderm grows in between the two structures and later gives place to a layer of homogeneous substance in which a few cells,
The Anterior Chamber Of The Eye
more numerous laterally than at the center, are embedded. Still later cells from the adjacent mesenchyme grow into the layer, which increases considerably in thickness, and blood-vessels also grow into that portion of it which is in contact with the outer surface of the lens. At this stage the interval between the surface ectoderm and the lens is occupied by a solid mass of mesodermal tissue (Fig. 278, co and tv), but as development proceeds, small spaces (ac) filled with fluid begin to appear toward the inner portion of the mass, and these, increasing in number and size, eventually fuse together to form a single cavity which divides the mass into an inner and an outer portion. The cavity is the anterior chamber of the eye, and it has served to separate the cornea (co) from the tunica vasculosa lentis (tv) , and, extending laterally in all directions, it also separates from the cornea the mesenchyme which rests upon the marginal portion of the optic cup and constitutes the stroma of the iris. Cells arrange themselves on the corneal surface of the cavity to form a continuous endothelial layer, and the mesenchyme which forms the peripheral boundary of the cavity assumes a fibrous character and forms the ligamentum pectinatum iridis, among the fibers of which cavities, known as the spaces of Fontana (Fig. 277, sf), appear. Beyond the margins of the cavity the corneal tissue is directly continuous with the sclerotic, beneath the margin of which is a distinctly thickened portion of mesenchyme resting upon the ciliary processes and forming the stroma of the ciliary body, as well as giving rise to the muscle tissue which constitutes the ciliary muscle (Figs. 277 and 278, mc).
Fig. 278. - Transverse Section through the Ciliary Region of a Pig Embryo or 23 MM. ac, Anterior chamber of the eye; co, cornea; ec, ectoderm; I, lens; mc, ciliary muscle; p, pigment layer of the optic cup; r, retinal layer; tv, tunica vasculosa lentis. - (Angelucci.)
The ectoderm which covers the outer surface of the eye does not proceed beyond the stage when it consists of several layers of cells, and never develops a stratum corneum. In the corneal region it rests directly upon the corneal tissue, which is thickened slightly upon its outer surface to form the anterior elastic lamina; more peripherally, however, a quantity of loose mesodermal tissue lies between the ectoderm and the outer surface of the sclerotic, and, together with the ectoderm, forms the conjunctiva (Fig. 277, cj).
The Development of the Accessory Apparatus of the Eye. - The eyelids make their appearance at an early stage as two folds of skin, one a short distance above and the other below the cornea. The center of the folds is at first occupied by indifferent mesodermal tissue, which later becomes modified to form the connective tissue of the lids and the tarsal cartilage, the muscle tissue probably secondarily growing into the lids as a result of the spreading of the platysma over the face, the orbicularis oculi apparently being a derivative of that sheet of muscle tissue.
At about the beginning of the third month the lids have become sufficiently large to meet one another, whereupon the thickened epithelium which has formed upon their edges unites and the lids fuse together, in which condition they remain until shortly before birth. During the stage of fusion the eyelashes (Fig. 279, h) develop at the edges of the lids, having the same developmental history as ordinary hairs, and from the fused epithelium of each lid there grow upward or downward, as the case may be, into the mesodermic tissue, solid rods of ectoderm, certain of which early give off numerous short lateral processes and become recognizable as the tarsal (Meibomian) glands (m), while others retain the simple cylindrical form and represent the glands of Moll. When the eyelids separate, these solid ingrowths become hollow by a breaking down of their
Fig. 279. - Section through the Margins of the Fused Eyelids in an Embryo^ of Six Months. i "1
h, Eyelash; //, lower lid; m, tarsal gland; mu, muscle bundle; ul, upper lid. Seidl.)
central cells, just as in the sebaceous and sudoriparous glands of the skin, the tarsal glands being really modifications of the former glands, while the glands of Moll are probably to be regarded as specialized sudoriparous glands.
A third fold of skin, in addition to^the two which produce the eyelids, is also developed in connection with the eye, forming the plica semilunaris. This is a rudimentary third eyelid, representing the nictitating membrane which is fairly well developed in many of the lower mammals and especially well in birds.
THE LACHRYMAL GLAND 467 The lachrymal gland is developed at about the third month as a number of branching outgrowths of the ectoderm into the adjacent mesoderm along the outer part of the line where the epithelium of the conjunctiva becomes continuous with that covering the inner surface of the upper eyelid. As in the other epidermal glands, the outgrowths and their branches are at first solid, later becoming hollow by the degeneration of their axial cells.
The naso-lachrymal duct is developed in connection with the groove which, at an early stage in the development (Fig. 62), extends
Fig. 280. - Diagram showing the Insertions of the Lachrymal Ducts in EMBRYOS OF 40 MM. AND 170 MM.. THE CaRUNCULA LaCRIMALIS BEING FORMED IN the Latter.
The eyelids are really fused at these stages but have been represented as separate â€¢ for the sake of clearness. - (Ask.) from the inner corner of the eye to the olfactory pit and is bounded posteriorly by the maxillary process of the first visceral arch. The epithelium lying in the floor of this groove thickens toward the begining of the sixth week to form a solid cord, which sinks into the subjacent mesoderm. From its upper end two outgrowths arise which become connected with the ectoderm of the edges of the upper and lower lids, respectively, and represent the lachrymal ducts, and, finally, the solid cord and its outgrowths acquire a lumen and a connection with the mucous membrane of the inferior meatus of the nasal cavity.
The inferior duct connects with the border of the eyelid some distance lateral to the inner angle of the eye, and between its opening and the angle a number of tarsal glands develop. The superior duct, on the other hand, opens at first close to the inner angle and later moves laterally until its opening is opposite that of the inferior duct. During this change the portion of the lower lid between the opening of the inferior duct and the angle is drawn somewhat upward, and, with its glands, forms a small reddish nodule, resting upon the plica semilunaris and known as the caruncula lacrimalis (Fig. 280).
G. Alexander: "Ueber Entwicklung und Bau des Pars inferior Labyrinthi der hdheren Saugethiere," Denkschr. kais. wissench. Acad. Wien, Math.-Naturw. Classe, lxx, 1901.
A. Angeltjcci: "Ueber Entwickelung und Bau des vorderen Uvealtractus der Verte braten," Archiv fur mikrosk. Anat., xix, 1881. F. Ask: " Ueber die Entwickelung der Caruncula lacrimalis beim Menschen, nebst Bemerkungen iiber die Entwickelung der Tranenrohrchen und der Meibom'schen Driisen," Anatom. Anzeiger, xxx, 1907.
F. Ask: "Ueber die Entwicklung der Lidrander, der Tranenkarunkel und der Nick haut beim Menschen, nebst Bemerkungen zur Entwicklung der Tranenabfuhrungswege," Anat. Hefte, xxxvi, 1908.
B. Baginsky: "Zur Entwickelung der Gehorschnecke," Archiv fur mikrosk. Anat., xxviii, 1886. I. Broman: "Die Entwickelungsgeschichte der Gehorknochelchen beim Menschen," Anat. Hefte, xi, 189S. S. Ramon y Cajal: "Nouvelles contributions a l'etude histologique de la retine," Journ. de I' Anat. et de la Physiol., xxxii, 1896.
G. Cirincione: "Ueber den gegenwartigen Stand der Frage hinsichtlich der Genese des Glaskorpers," Arch, fur Augenheilk., L, 1904. A. Contino: "Ueber Bau and Entwicklung des Lidrandes beim Menschen," Arch, fur Ophthalmol., lxvi, 1908.
A. Contino: "Ueber die Entwicklung der Karunkel und der plica semilunaris beim Menschen," Arch, fur Ophthalmol, lxxi, 1909. J. Disse: "Die erste Entwickelung der Riechnerven," Anat. Hefte, ix, 1897.
B. Fleischer: "Die Entwickelung der Tranenrohrchen bei den Saugetiere," Archiv fur Ophthalmol., lxii, 1906. H. Fuchs : " Bemerkungen iiber die Herkunft und Entwickelung der Gehorknochelchen bei Kaninchen-Embryonen (nebst Bemerkungen iiber die Entwickelung des Knorpelskeletes der beiden ersten Visceralbogen)," Archiv. fur Anat und Phys., Anat. Abth., Supplement, 1905. J. Graberg: "Beitrage zur Genese des Geschmacksorgans der Menschen," Morphol.
Arbeiten, vn, 1898. J. A. Hammar: "Zur allgemeinen Morphologie der Schlundspalten des Menschen.
Zur Entwickelungsgeschichte des Mittelohrraumes, des ausseren Gehorganges und des Paukenfelles beim Menschen," Anat. Anzeiger, xx, 1901.
J. A. Hammar: " Studien iiber die Entwicklung des Vorderdarms und einiger angrenz ender Organe," Arch, fur mikrosk. Anat., Lix, 1902. C. Heerfordt: "Studien iiber den Muse, dilatator pupilke sammt Angabe von gemeinschaftlicher Kennzeichen einiger Falle epithelialer Musculatur," Anat.
Hefte, xiv. J. Hegetschweiler: "Die embryologische Entwickelung des Steigbugels," Archiv fur Anat. und Physiol., Anat. Abth., 1898.
F. Hochstetter: "Ueber die Bildung der primitiven Choanen beim Menschen," Verhandl. Anat. Gesellsch., VI, 1892. W. His, Jr.: "Die Entwickelungsgeschichte des Acustico-Facialisgebietes beim Menschen," Archiv fur Anat. und Physiol., Anat. Abth., Supplement, 1897. A. von Kolliker: "Die Entwicklung und Bedeutung des Glaskorpers," Zeitschr.
fur wissensch. Zoolog., lxxvi, 1904. P. Lang: "Zur Entwicklung des Tranenausfiihrsapparates beim Menschen," Anat.
Anzeiger," xxxvni, 1911.
G. Leboucq: " Contribution a. l'etude de l'histogenese de la retine chez les mammiferes," Arch. Anat., Microsc, x, 1909. V. von Mihalkovicz: "Nasenhohle und Jacobsonsches Organ. Eine morphologische Studie." Anat. Hefte. xi, 1898. J. L. Paitlet: "Contribution a l'etude de l'organe de Jacobson chez l'embryon humain," Bibliogr. Anat., xvii, 1907. P. van Pee: "Recherches sur l'origine du corps vitre," Archives de Biol., xix, 1902. C. Rabl: "Ueber den Bau und Entwickelung der Linse," Zeitschrift fur wissensch.
Zoologie, lxiii and lxv, 1898; lxviii, 1899. A. Robinson: "On the Formation and Structure of the Optic Nerve and Its Relation to the Optic Stalk," Journal of Anat. and Physiol., xxx, 1896. G. Speciale-Cirincione: "Ueber die Entwicklung der Tranendriise beim Menschen " Arch.filr Ophthalmol., LXIX, 1908. J. P. Schaefeer: "The Genesis and Development of the Nasolacrimal Passages in Man," Amer. Journ. Anat., xm, 1912. G. L. Streeter: "On the Development of the Membranous Labyrinth and the Acoustic and Facial Nerves in the Human Embryo," Amer. Journ. of Anat.
vi, 1907. N. van der Stricht: "L'histogenese des parties constituantes du neuroepithelium acoustique, des taches et des cretes acoustiques et de l'organe de Corti " Arch.
de Biol., xxiii, 1908. A. Szili: "Zur Anatomie und Entwickelungsgeschichte der hinteren Irisschichten mit besonderer Beriicksichtigung des Musculus sphincter iridis des Menschen " Anat. Anzeiger, xx, 1901. A. Szili: "Ueber das Entstehen eines fibrillares Stutzgewebes im Embryo und dessen Verhaltnis zur Glaskorperfrage," Anat. Hejte, xxxv, 190S. F. Tuckerman: "On the Development of the Taste Organs in Man," Journal of Anat.
and Physiol., xxiv, 1889. R. Versari: "Ueber die Entwicklung der Blutgefasse des menschlichen Au^es " Anat. Anzeiger, xxxv, 1909.
Chapter XVII. Post-Natal Development
In the preceding pages attention has been directed principally to the changes which take place in the various organs during the period before birth, for, with a few exceptions, notably that of the liver, the general form and histological peculiarities of the various organs are acquired before that epoch. Development does not, however, cease with birth, and a few statements regarding the changes which take place in the interval between birth and maturity will not be out of place in a work of this kind.
The conditions which obtain during embryonic life are so different from those to which the body must later adapt itself, that arrangements, such as those connected with the placental circulation, which are of fundamental importance during the life in utero, become of little or no use, while the relative importance of others is greatly diminished, and these changes react more or less profoundly on all parts of the body. Hence, although the post-natal development consists chiefly in the growth of the structures formed during earlier stages, yet the growth is not equally rapid in all parts, and indeed in some organs there may even be a relative decrease in size. That this is true can be seen from the annexed figure (Fig. 281), which represents the body of a child and that of an adult man drawn as of the same height. The greater relative size of the head and upper part of the body in the child is very marked, and the central point of the height of the child is situated at about the level of the umbilicus, while in the man it is at the symphysis pubis.
That there is a distinct change in the geometric form of the body during growth is also well shown by the following consideration. (Thoma). Taking the average height of a new-born male as 500 mm., and that of a man of thirty years of age as 1686 mm., the height of the body will have increased from birth to adolescence - v _â– = 3.37 times. The child will weigh 3.1 kilos and the man 5:: 66.1 kilos, and if the specific gravity of the body with the included gases be taken in the one case as 0.90 and in the other as 0.93, then the volume of the child's body will be 3.44 liters and that of the man's 71.08 liters, and the increase in volume will be - - =20.66.
Fig. 281. - Child ast> }vL\x Drawn as of th* " Growth of the Brain, " Contemporary Science Series Sons.)
If the increase in volume had taken place without any alteration in the geometric form of the body, it should be equal to the cube of the increase in height; this, however, is 3-37 s =38.27, a number wellnigh twice as large as the actual increase.
But in addition to these changes, which are largely dependent upon differences in the supply of nutrition, there are others associated with alterations in the general metabolism of the body. Up to adult life the constructive metabolism or anabolism is in excess of the destructive metabolism or katabolism, but the amount of the excess is much greater during the earlier periods of development and gradually diminishes as the adult condition is approached. That this is true during intrauterine life is shown by the following figures, compiled by Donaldson:
Age in Weeks
Weight in Grams
Age in Weeks
Weight in Grams
o . 0006
From this table it may be seen that the embryo of eight weeks is six thousand six hundred and sixty-seven times as heavy as the ovum from which it started, and if the increase of growth for each of the succeeding periods of four weeks be represented as percentages, it will be seen that the rate of increase undergoes a rapid diminution after the sixteenth week, and from that on diminishes gradually but less rapidly, the figures being as follows :
Periods of Weeks
Periods of Weeks
8-12 12-16 16-20 20-24
400 500 137 123
24-28 28-32 32-36 36-40
92 39 32 45
That the same is true in a general way of the growth after birth may be seen from the following table, representing the average weight of the body in English males at different years from birth up to twenty-three (Roberts), and also the percentage rate of increase.
Number of Cases
Weight in Kilograms
Certain interesting peculiarities in post-natal growth become apparent from an examination of this table. For while there is a general diminution in the rate of growth, yet there are marked irregularities, the most noticeable being (i) a rather marked diminution during the eleventh and twelfth years, followed by (2) a rapid
- From a comparison with other similar tables there is little doubt but that the weight given above for the second year is too high to be accepted as a good average
1 Z 3 * 5 6 f a 9 ID ft 12 13 14 1$ 16 17 19
2 3 4-5 6 7 8 9 10 11 12 13 J& I
5 17 18
Age Lbs/4 " 12 " 10 â€¢â– 8 " 6 " * ' Z
Fig. 282. - Curves Showing the Annual Increase in Weight in (I) Boys and (II) Girls.
The faint line represents the curve from British statistics, the dotted line that from American (Bowditch), and the heavy line the average of the two. Before the sixth year the data are unreliable. - (Stephenson.) acceleration which reaches its maximum at about the sixteenth year and then very rapidly diminishes. These irregularities may be more Consequently the percentage increase for the second year is too high and that for the third year too low.
It may be mentioned that the weights in the original table are expressed in pounds avoirdupois and have been here converted into kilograms, and further the figures representing the percentage increase have been added.
clearly seen from the charts on page 474, which represent the curves obtained by plotting the annual increase of weight in boys (Chart I) and girls (Chart II). The diminution and acceleration of growth referred to above are clearly observable and it is interesting to note that they occur at earlier periods in girls than in boys, the diminution occurring in girls at the eighth and ninth years and the acceleration reaching its maximum at the thirteenth year.
Considering, now, merely the general diminution in the rate of growth which occurs from birth to adult life, it becomes interesting to note to what extent the organs which are more immediately associated with the metabolic activities of the body undergo a relative reduction in weight. The most important of these organs is undoubtedly the liver, but with it there must also be considered the thyreoid and thymus glands, and probably the suprarenal bodies. In all these organs there is a marked diminution in size as compared with the weight of the body, as will be seen from the following table (H. Vierordt), which also includes data regarding other organs in which a marked relative diminution, not in all cases readily explainable, occurs.
ABSOLUTE WEIGHT IN GRAMS.
New-born and Adult.
â€ž . Spmal Brain _; . Cord
PERCENTAGE WEIGHT OF ENTIRE BODY New-born and Adult.
4-57 2 -57
12 .29 2 .16
Recent observations by Hammar render necessary some modification of the figures given for the thymus in the above table. He finds the average weight of the gland at birth to be 13.26 grms., and that the weight increases up to puberty, averaging 37.52 grms. between the ages of 11 and 15. After that period it gradually diminishes, falling to 16.27 g rm sbetween 36 and 45, and to 6.0 grms. between 66 and 75. Expressed in percentage of the body weight this gives a value in the new-born of 0.42 and in an individual of 50 years of 0.02, a difference much more striking than that shown in Vierordt's table.
It must be mentioned, however, that the gland is subject to much individual variation, being largely influenced by nutritive conditions.
The remaining organs, not included in the tables given above, when compared with the weight of the body, either show an increase or remain practically the same.
ABSOLUTE WEIGHT IN GRAMS. New-born and Adult.
Skin and Subcutaneous Tissues
Stomach and Musculature , T Intestines
776.5 65 28,732.0 1,364
PERCENTAGE OF BODY-WEIGHT. New-born and Adult.
Skin and Subcutaneous Tissues
Stomach and Intestines
2 5-05 43-40
2 . 1 2 .06
0. 11 015
From this table it will be seen that the greatest increment of weight is that furnished by the muscles, the percentage weight of which is one and three-fourths times as great in the adult as in the child. The difference does not, however, depend upon the differentiation of additional muscles; there are just as many muscles in the new-born child as in the adult, and the increase is due merely to an enlargement of organs already present. The percentage weight of the digestive tract, pancreas, and lungs remains practically the same, while in the case of the skeleton there is an appreciable increase, and in that of the skin and subcutaneous tissue a slight
Fig. 283. - Longitudinal Section through the Sacrum of a New-born Female Child. - (Fehling.)
diminution. The latter is readily understood when it is remembered that the area of the skin, granting that the geometric form of the body remains the same, would increase as the square of the length, while the mass of the body would increase as the cube, and hence in comparing weights the skin might be expected to show a diminution even greater than that shown in the table.
The increase in the weight of the skeleton is due to a certain extent to growth, but chiefly to a completion of the ossification of the cartilage largely present at birth. A comparison of the weights of this system of organs does not, therefore, give evidence of the many changes of form which may be perceived in it during the period under consideration, and attention may be drawn to some of the more important of these changes.
In the spinal column one of the most noticeable peculiarities observable in the new-born child is the absence of the curves so characteristic of the adult. These curves are due partly to the weight of the body, transmitted through the spinal column to the hipjoint in the erect position, and partly to the action of the muscles, and it is not until the erect position is habitually assumed and the musculature gains in development that the curvatures become pronounced. Even the curve of the sacrum, so marked in the adult, is but slight in the new-born child, as may be seen from Fig. 283, in which the ventral surfaces of the first and second sacral vertebrae look more ventrally than posteriorly, so that there is no distinct promontory.
But, in addition to the appearance of the curvatures, other changes also occur after birth, the entire column becoming much more slender and the proportions of the lumbar and sacral vertebrae becoming quite different, as may be seen from the following table (Aeby) :
Lengths Of The Vertebral Regions Expressed As Percentages Of The Entire Column
25.6 23-3 20.3 19.7 22 .1
47-5 46.7 45-6 47.2 46.6
Male 2 years
Male 5 years
Male 1 1 years Male adult
The cervical region diminishes in length, while the lumbar gains, the thoracic remaining approximately the same. It may be noticed, furthermore, that the difference between the two variable regions is greater during youth than in the adult, a condition possibly associated with the general more rapid development of the lower portion of the body made necessary by its imperfect development during fetal life. The difference is due to changes in the vertebrae, the intervertebral disks retaining approximately the same relative thickness throughout the period under consideration.
The form of the thorax also alters, for whereas in the adult it is barrel-shaped, narrower at both top and bottom than in the middle, in the new-born child it is rather conical, the base of the cone being below. The difference depends upon slight differences in the form and articulations of the ribs, these being more horizontal in the child and the opening of the thorax directed more directly upward than in the adult.
As regards the skull, the processes of growth are very complicated. Cranium and brain react on one another, and hence, in harmony with the relatively enormous size of the brain at birth, the cranial cavity has a relatively greater volume in the child than in the adult. The fact that the entire roof and a considerable part of the sides of the skull are formed of membrane bones which, at birth, are not in sutural contact with one another throughout, gives opportunity for considerable modifications, and, furthermore, the base of the skull at the early stage still contains a considerable amount of unossified cartilage. Without entering into minute details, it may be stated that the principal general changes which the skull undergoes in its post-natal development are (i) a relative elongation of its anterior portion and (2) an increase in the relative height of the maxillae.
If a line be drawn between the central points of the occipital condyles, it will divide the base of the skull into two portions, which in the child's skull are equal in length. The portion of the skull in front of a similar line in the adult skull is very much greater than that which lies behind, the proportion between the two parts being 5:3, against 3:3 in the child (Froriep). There has, therefore, been a decidedly more rapid growth of the anterior portion of the skull, a growth which is asssociated with a corresponding increase in the dorso-ventral dimensions of the maxillae. These bones, indeed, play a very important part in determining the proportions of the skull at different periods. They are so intimately associated with the cranial portions of the skull that their increase necessitates a corresponding increase in the anterior part of the cranium, and their increase in this direction stands in relation to the development of the teeth, the eight teeth which are developed in each maxilla (including the premaxilla) in the adult requiring a longer bone than do the five teeth of the primary dentition, these again requiring a greater length when completely developed than they do in their immature condition in the new-born child.
Fig. 284. - Skull of a New-born Child and of an Adult Man, Drawn as of Approximately the Same Size. - (Henke.)
But far more striking than the difference just described is that in the relative height of the cranial and facial regions (Fig. 284). It has been estimated that the volumes of the two portions have a ratio of 8: 1 in the new-born child, 4: 1 at five years of age, and 2:1 in the adult skull (Froriep) , and these differences are due principally to changes in the vertical dimensions of the maxillae. As with the increase in length, the increase now under consideration is, to a certain extent at least, associated with the development of the teeth, hese structures calling into existence the alveolar processes which ,re practically wanting in the child at birth. But a more important actor is the development of the maxillary sinuses, the practically olid bodies of the maxillae becoming transformed into hollow shells, rhese cavities, together with the sinuses of the sphenoid and frontal >ones, which are also post-natal developments, seem to stand in elation to the increase in length of the anterior portion of the skull, erving to diminish the weight of the portion of the skull in front Â»f the occipital condyles and so relieving the muscles of the neck of a onsiderable strain to which they would otherwise be subjected.
These changes in the proportions of the skull have, of course, nuch to do with the changes in the general proportions of the face. 3ut the changes which take place in the mandible are also imporant in this connection, and are similar to those of the maxillae in leing associated with the development of the teeth. In the new10m child the horizontal ramus is proportionately shorter than in he adult, while the vertical ramus is very short and joins the Lorizontal one at an obtuse angle. The development of the teeth if the primary dentition, and later of the three molars, necessitates ,n elongation of the horizontal ramus equivalent to that occurring n the maxillae, and, at the same time, the separation of the alveolar â€¢orders of the two bones requires an elongation of the vertical ramus f the condyle is to preserve its contact with the mandibular fossa, ,nd this, again, demands a diminution of the angle at which the ami join if the teeth of the two jaws are to be in proper apposition.
In the bones of the appendicular skeleton secondary epiphysial enters play an important part in the ossification, and in few are hese centers developed prior to birth, while the union of the epiphyes to the main portions of the bones takes place only toward maurity. The dates at which the various primary and secondary enters appear, and the time at which they unite, may be seen from he following table:
POST-NATAL DEVELOPMENT UPPER EXTREMITY.
Appearance of Secondary
(At sternal end) 17th year.
8th week. <.
2 acromial 15th year.
2 on vertical border 16th year.
> 20th year.
1 st year.
15 th year.
Head 1st year.
Great tuberosity 3d year.
> 20th year.
Lesser tuberosity 5th year.
â– jth week.
Inner condyle 5th year.
1 8th year.
Capitellum 3d year.
Trochlea 10th year.
[â€¢ 17 th year.
Outer condyle 14th year.
Olecranon 10th year.
Distal epiphysis 4th year.
1 8th year.
Proximal epiphysis 5th year.
17 th year.
Distal epiphysis 2d year.
Triquetrum . . .
12 th year.
Metacarpals . . .
17 th-! 8th years.
The dates in italics are before birth.
POST-NATAL DEVELOPMENT LOWER EXTREMITY.
Appearance of Secondary Fusion of
Crest 15th year.
Anterior inferior spine 15 th year.
â– 22d year.
Tuberosity 15th year.
Crest 1 Sth year.
Cartilage appears at 4th month, ossification in 3d year.
Head 1st year.
20 th year.
â– jth week.
Great trochanter 4th year. Lesser trochanter 13 th- 14th year. Condyle gth month.
19 th year. 1 Sth year. 2 1 st year.
Head end of gth month. Distal end 2d year.
2ist-2 5thyear. 1 Sth year.
Upper epiphysis 5th year. Lower epiphysis 2d year.
21st year. 20th year.
1 6 th year.
A few days after birth.
1 st year.
The dates in italics are before birth.
So far as the actual changes in the form of the appendicular bones are concerned, these are most marked in the case of the lower limb. The ossa innominata alter somewhat in their proportions after birth, a fact which may conveniently be demonstrated by considering the changes which occur in the proportions of the pelvic diameters, although it must be remembered that these diameters are greatly influenced by the development of the sacral curve. Taking the conjugate diameter of the pelvic brim as a unit for comparison, the antero-posterior (dorso-ventral) and transverse diameters of the child and adult have the proportions shown in the table on the opposite page (Fehling).
It will be seen from this that the general form of the pelvis in the new-born child is that of a cone, gradually diminishing in diameter from the brim to the outlet, a condition very different from what obtains in the adult. Furthermore, it is interesting to note
(Conjugata vera . Transverse >, f Antero-posterior rt 1 U y Transverse -^ ( Antero-posterior O Transverse
1.294 1. 18 1. 14 1 .07 1 -153
that sexual differences in the form of the pelvis are clearly distinguishable at birth; indeed, according to Fehling's .observations, they become noticeable during the fourth month of intrauterine development.
The upper epiphysis of the femur is entirely unossified at birth and consists of a cartilaginous mass, much broader than the rather slender shaft and possessing a deep notch upon its upper surface (Fig. 285). This notch marks off the great trochanter from the head of the bone, and at this stage of development there is no neck, the head being practically sessile. As development proceeds the inner upper portion of the shaft grows more rapidly than the outer portion, carrying the head away from the great trochanter and forming the neck of the bone. The acetabulum is shallower at birth than in the adult and cannot contain more than half the head of the femur; consequently the articular portion of the head is much less extensive than in the adult.
It is a well-known fact that the new-born child habitually holds the feet with the soles directed toward one another, a position only reached in the adult with some difficulty, and associated with this supination or inversion there is a pronounced extension of the foot (i. e., flexion upon the leg as usually understood; see p. 102), it being difficult to flex the child's foot beyond a line at right angles with the axis of the leg. These conditions are due apparently to the extensor and tibialis muscles being relatively shorter and the opposing muscles relatively longer than in the adult, and with the elongation or shortening, as the case may be, of the muscles on the assumption
Fig. 2S5. - Longitudinal Sections of the Head of the Femur of (.4) New-born Child and (B) a Later Stage of Development.
ep, Epiphysial center for the head; h, head; /, trochanter. - (Henke.) of the erect position, the bones in the neighborhood of the anklejoint come into new relations to one another, the result being a modification of the form of the articular surfaces, especially of the talus (astragalus). In the child the articular cartilage of the trochlear surface of this bone is continued onward to a considerable extent upon the neck of the bone, which comes into contact with the tibia in the extreme extension possible in the child. In the adult, however, such extreme extension being impossible, the cartilage upon the neck gradually disappears. The supination in the child brings the talus
in close contact with the inner surface of the calcaneus and with the sustentaculum tali; with the alteration of position a growth of these portions of the calcaneus occurs, the sustentaculum becoming higher and broader, and so becoming an obstacle in the way of supination in the adult. At the same time a greater extent of the outer surface of the talus comes into contact with the lateral malleolus, with the result that the articular surface is considerably increased on that portion of the bone. Marked changes in the form of the talo-navicular articulation also occur, but their consideration would lead somewhat further than seems desirable.
C. Aeby: "Die Altersverschiedenheiten der menschlichen Wirbelsaule." Archiv fur Anal, und Physiol., Anat. Abth., 1879. W. Camerer: " Utersuchungen iiber Massenwachsthum und Langen wachsthum der Kinder," Jahrbuchfiir Kinderheilkunde, xxxvi, 1893. H. H. Donaldson: "The Growth of the Brain," London, 1895. H. Fehling: "Die Form des Beckens beim Fotus und Neugeborenen und ihre Bezie hung zu der beim Erwachsenen," Archiv fur Gynakol., x, 1876. H. Friedenthal: " Das Wachsthum des Korpergewichtes des Menschen und anderer Saugethiere in verschiedenen Lebensaltern," Zeit. allgem. Physiol., ix, 1909. J. A. Hammar: "Ueber Gewicht, Involution und Persistenz der Thymus im Post fotalleben des Menschen," Archiv fur Anat. und Phys., Anat. Abth., Supplement, 1906. W. Henke: " Anatomie des Kindersalters," Handbuch der Kinder krankheiten (Cerhardt) , Tubingen, 1881. C. Hennig: "Das kindliche Becken," Archiv fur Anat. und Physiol., Anat. Abth., 1880. C. Huter: "Anatomische Studien an den Extremitatengelenken Neugeborener und Erwachsener," Archiv fur patholog. Anat. und Physiol., xxv, 1862. W. Stephenson: "On the Relation of Weight to Height and the Rate of Growth in Man," TheLancet, 11, 1888. R. Thoma: " Untersuchungen iiber die Grosse und das Gewicht der anatomischen Bestandtheile des menschlichen Korpers," Leipzig, 1882. H. Vierordt: "Anatomische, Physiologische und Physikalische Daten und Tabellen," Jena, 1893. H. Welcker: "Untersuchungen iiber Wachsthum und Bau des menschlichen Schadels," Leipzig, 1862.
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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.
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