Book - Embryology of the Pig 4
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Patten BM. Embryology of the Pig. (1951) The Blakiston Company, Toronto.
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Patten BM. Developmental defects at the foramen ovale. (1938) Am J Pathol. 14(2):135-162. PMID 19970381
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Chapter 4. The Process of Cleavage and the Formation and Early Differentiation of the Germ Layers
Cleavage
The series of cell divisions which, immediately after fertilization, follow upon one another in close succession are known as the cleavage or segmentation divisions. In the various groups of animals cleavage differs much in detail, but these differences are not as fundamental as at first glance they might appear. They are due almost entirely to the varying amount of yolk stored in the egg cells.
A mitotic division, whether it be one of the cleavage divisions of an ovum or the division of some other cell, is carried out by the active protoplasm of the cell. The food material stored in the cytoplasm of an egg cell is non-living and inert. This deutoplasm, as it is called, plays no plart in mitosis except as it exerts a local retarding effect by the mechanical impediment it offers to the process.
Figure 12 shows diagrammatically how the first cleavage division is carried out in three types of eggs having different relative amounts of yolk, and different distributions of the yolk in the cytoplasm. In the egg of such forms as Amphioxus the yolk is relatively meager in amount and fairly uniformly distributed throughout the cytoplasm. An ovum w^th such a yolk distribution is termed isolecithal {homolecithal). An isolecithal egg undergoes a type of cleavage which is essentially an unmodified mitosis. The yolk is not sufficient in amount, nor sufficiently localized, to alter the usual mode of cell division.
In Amphibia the ovum contains a considerable amount of yolk and the accumulation of the yolk at one pole has crowded the nucleus and the active cytoplasm of the ovum toward the opposite pole. An egg in which the yolk is thus concentrated is termed telolecithal. The region of the egg in which the yolk is accumulated is designated as its vegetative pole and the opposite region where the nucleus and most of the active cytoplasm are situated is called its animal pole.
Cleavage in all types of eggs is initiated by the mitotic division of the nucleus. Division of the cytoplasm follows nuclear division. In telolecithal eggs the cytoplasm at the animal pole where there is little or no yolk divides promptly, but where the yolk mass is encountered the process is greatly retarded. So slowly, in fact, is the division of the yolk accomplished that succeeding cell divisions begin at the animal pole of the egg before the first cleavage is completed at the vegetative pole.
Fig. 12. Schematic diagrams to indicate the effect of yolk on the first cleavage division.
The eggs of birds are also telolecithal, but the amount of yolk which they contain is both relatively and actually much greater than that in Amphibian eggs. Cleavage in birds’ eggs begins as it does in the eggs of Amphibia, but the mass of the inert yolk material in them is so great that the yolk is not divided. The process of segmentation is limited to the small disk of protoplasm lying on the surface of the yolk at the animal pole, and is for this reason referred to as discoidal cleavage. The fact that the whole egg is not divided is indicated by designating the process as partial (meroblastic) cleavage in distinction to the complete (holoblastic) cleavage seen in eggs containing less yolk. The cells formed in the process of segmentation are known as blastomeres whether they are completely separated, as is the case in holoblastic cleavage, or only partially separated, as in meroblastic cleavage.
In mammalian ova the deutoplasmic content is exceedingly small, correlated with the fact that the embryo at an early stage in its development draws upon the uterine circulation of the mother for its nutrition. For this reason the cleavage of mammalian ova reverts to the simple type seen in primitive forms with a scanty and uniformly distributed yolk content. It is practically an unmodified mitosis. To put it into the usual technical terms of comparative embryology, we are dealing with the equal holoblastic cleavage of an isolecithal egg.
Through the pioneer work of Asshe ton and the subsequent extensive work of Streeter and Heuser the pig is one of the few mammals from which anything like a complete series of cleavage stages has been secured. Like other mammalian ova, that of the pig shows but a scanty amount of deutoplasm. This deutoplasm, in the pig ovum, consists chiefly of fat droplets scattered through the peripheral part of the cytoplasm. In sectioned specimens the fat has been dissolved by the alcohols through which the material is passed preparatory to its embedding but it leaves in the fixed cytoplasm numerous vacuoles as a record of its presence (Fig. 14, A). The uniformity of the distribution of the deutoplasm gives no clear-cut differentiation of a 'Vegetative pole†such as is afforded by the yolk in the case of Amphibian and Sauropsidan ova. The point at which the polar bodies are given off, however, establishes the animal pole and gives us thereby a basis of orientation. As is the general case, the first cleavage spindle forms at right angles to an imaginary axis drawn through the ovum from animal to vegetative pole (Fig. 13, B). When the first cleavage division of the ovum is completed the plane of separation between the resulting blastomeres, being in the equator of the spindle, lies in the imaginary animal-vegetative axis of the ovum (cf. Figs. 13, C, and 14, A).
Fig. 13. Schematic diagrams showing the process of cleavage as it takes place in ova having only a small amount of yolk in their cytoplasm. (After William Patten.) Note that the blastomeres are separated from one another promptly and completely. (Patten: '‘Early Embryology of the Chick,†The Blakiston Company.)
Fig. 14. Cleavage of the pig ovum. (A, B, and D, drawn from preparations loaned by Streeter and Heuser; C, after Assheton.) (All figures X 300.)
A, Two-cell stage in section. Specimens secured from the oviduct of a sow killed 2 days, hours after copulation.
B, Four-cell stage in section. Probable age about 23^ days.
C, A morula of about 16 cells. Drawn from unsectioned specimen, probable age about 33^^ days.
D, Blastula stage, drawn from an unsectioned specimen secured from the uterus of a sow killed 4J^^ days after copulation. Note the lighter central area indicating the beginning of the formation of the segmentation cavity (blastocoele) by cell rearrangement (cf. Fig. 15).
A second cleavage spindle is formed in each of the first two blastomeres almost as soon as they are established. The second spindles form at right angles to the first (Fig. 13, C), and the second cleavage planes are consequently perpendicular to the first cleavage plane.
Further cleavage divisions follow one another in such rapid succession that the growth interval usually intervening between succeeding mitoses is curtailed. In consequence, the individual blastomeres in each succeeding generation become smaller and smaller (cf. Fig. 14, A-D). Since the zona pellucida persists intact throughout the period of cleavage, the blastomeres are forced to dispose themselves within its spheroidal cavity. After several segmentation divisions have taken place, the resultant group of blastomeres appears much like a solid ball of cells suggestive of a mulberry or a blackberry. The embryo in this condition is said to be in the morula (translated == little mulberry) stage (Fig. 14, C).
After a characteristic morula has been formed, the term cleavage is not ordinarily applied to the cell divisions which occur. The inference should not be drawn that active cell division ceases or even that il is retarded in rate. On the contrary it continues with unabated rapidity. But processes of segregation and differentiation even thus early begin to make their appearance and the term cleavage, which implies merely increase in cell numbers through repeated cell divisions, ceases adequately to characterize the phenomena.
The Blastula Stage
Terms designating ‘^stages of development†are convenient in discussing the progress of events, and the relative uniformity which has gradually been established in their usage is a great aid to mutual understanding. It should be borne in mind, however, that the delimitation of “stages†is purely arbitrary, for development is a cqntinuous process and one phase merges into another without any real point of demarcation.
When the blastomeres of a morula begin to be rearranged and organized about a central cavity (Fig. 14, D), we say that the morula is becoming a blastula or that the embryo is entering the blastula stage. The newly formed cavity within it is called the segmentation cavity or blastocoele (Fig. 15). Because of the large size attained by the blastocoele in mammals, mammalian embryos in the blastula stage are commonly called blastocysts, or blastodermic vesicles.
In the formation of the blastocyst an internal cluster of cells is established at one pole. This, for want of a better term, has been called the inner cell mass (Fig. 15). Although it cannot be carried through in all details, the general distinction may be made that the inner cell mass is destined to be concerned primarily with the formation of the embryonic body, whereas the thin outer wall of the blastocyst contributes, not to the make-up of the embryo, but to the formation of certain membranes which acquire intimate relations with the uterus of the mother and are concerned with the absorption of food. For this reason the thin layer of cells which constitutes the blastocyst wall is called the trophoblast (Fig. 15).
Fig. 15. Three stages of the blastodermic vesicle (blastocyst) of the pig, drawn from sections to show the formation of the inner cell mass. (A, B, from embryos in the Carnegie Collection; C, after Comer — all X 375.) A, Removed from uterus of sow days after copulation (cf. Fig. 14, D). B, Copulation age, 6 days, 1 hours. C, Copulation age, 6 days, 20 hours.
The Formation of the Entoderm
The blastocoele, once established, enlarges rapidly so that the inner cell mass appears propnjrtionately smaller because it occupies relatively less and less of the lumen of the blastocyst (Fig. 15). Actually, however, its cells are not only proliferating rapidly in situ but some of them apparently push out of the mass into the blastocoele. These are the first of the entoderm cells (Fig. 16, A). They are increased in numbers very rapidly after their first appearance and soon come to constitute a second complete layer inside the original outer layer of the blastocyst (Fig. 16, B, C). The internal lumen bounded by the entoderm is known as the primitive gut (archenteron) .
While the entoderm is being established another important change is taking place in the blastocyst. Originally the cell cluster constituting the inner cell mass was, as its name implies, completely inside the trophoblast. During the period of entoderm formation the overlying trophoblast degenerates and the original inner cell mass comes to lie at the surface, constituting part of the outer wall of the blastocyst (Fig. 16). At the same time its cells proliferate rapidly and become aggregated into a disk-shaped, thickened area sharply differentiated from the adjoining trophoblast. We now call this differentiated area arising from the inner cell mass the embryonic disk (Fig. 16, C). In fresh embryos viewed under a dissecting microscope the embryonic disk appears as a whitish area of markedly greater density than adjoining portions of the blastocyst where the wall consists only of the trophoblast and the even thinner entodermal layer within it (Fig. 17).
Fio. 16. Sections of pig blastocysts showing the first appearance and subsequent rapid extension of the entoderm. (From embryos in the Carnegie Collection.) Left, detailed drawings of inner cell mass (X 375). Right, sketches of same sections entire. The approximate age of the embryos represented ranges from 7 to 8 days.
Fig. 17. Blastodermic vesicles (blastocysts) of the pig at the beginning of elongation (about 8 days old). All seven embryos were removed from the same uterus. (Carnegie Collection, C264, after a photograph (X 3) by Dr. Heuser.)
Fig. 18. a litter of embryos slightly older than those of the preceding figure, showing the exceedingly rapid elongation of the blastocyst which occurs at this stage (about the ninth day) of development. (Carnegie Collection, C213, after a photograph (X 234) t>y Dr. Heuser.)
The Elongation of the Blastocyst
Shortly after the entoderm has been established as a definite layer, the blastocyst as a whole undergoes a striking change in shape. From an approximately spherical vesicle it becomes converted into a tubular sac. The rapidity with which this alteration in form takes place is astonishing. The embryos shown in figure 17 are all from the same litter and consequently very nearly of the same age. The least developed embryo in the group departs but slightly from its originally spherical shape, yet the most dev^eloped has already attained a length twelve times its diameter.
Figure 18 shows a slightly older litter. Even the most elongated embryo in this group has by no means attained its maximum length. In another day or two of development all these blastocysts would have become of thread-like thinness and approximately a meter long. A small region near the middle of the thread remains somewhat less attenuated and there the embryonic disk is located. The disk itself is practically unaffected in this process of elongation which involves the trophoblast a^d to a somewhat lesser extent the layer of entoderm lying within it. The significance of this increase in extent of the extraembryonic portion of the blastocyst will be appreciated when we see the part it plays in the formation of the extensive membranes through which the embryo draws upon the uterine circulation of the mother for its food supply (Chap. 6).
The Formation of the Primitive Streak. At about the same time that the blastocyst begins elongation, a local differentiation occurs in the embryonic disk which presages the formation of the primitive streak. Sections through the disk show at one part of its margin a heightened rate of cell proliferation accompanied by a definite increase in thickness (Fig. 19, A). Interpreting this thickening in the light of later developments it is possible for us to say that its appearance definitely establishes the longitudinal axis of the embryo. The thickening occurs at the part of the disk which is destined to become differentiated into the caudal end of the embryo.
Fig. 19. Longitudinal sections of the embryonic disk of the pig during the ninth day of development, showing three stages in the origin of the mesoderm. (Projection drawings ( X 200) from sections of embryos in the Carnegie Collection.)
In dorsal views of an entire embryo the thickened area when it first appears is crescentic in shape, with its convexity indicating the caudal extremity of the embryonic disk, and its horns spreading out over the greater part of the caudal half of the margin of the disk (Fig. 20, A). At this stage of development the embryonic disk apparently undergoes rapid concrescence caudally. That is, while the anterior margin of the disk is spreading out radially in a manner typical of uniform rate and unspccialized direction of growth, the posterior margins grow at an accelerated rate toward a point of convergence at the caudal extremity of the disk. (See arrows in Fig. 20, B.) This tends at the same time to lengthen the disk itself cephalocaudally, and to crowd the horns of the crescentic thickened area toward the mid-line. Further progress of this convergent differential growth changes the originally crescentic thickened area of the embryonic disk to an oval (Fig. 20, D) and then pulls it out into a band lying in the long axis of the embryo (Fig. 20, E-G). This thickened longitudinal band is known as the primitive streak.
The Primitive Streak as a Growth Center
The change in shape and position undergone by the originally crescentic area of the embryonic disk in no way retards its activity as a growth center. We find this area, throughout its transformation and later when it has become the primitive streak, still a region of rapid proliferation from which newly formed cells are constantly being pushed forth to take their part in the formation of the rapidly expanding body of the embryo. It seems not unlikely that the formation of the groove in the primitive streak (Fig. 23) is a local structural modification entailed by the rapid emigration of cells from this region (Fig. 22, E).
The Formation of the Mesoderm
The most striking manifestation of the activity of the cells in the primitive streak region is the formation of the mesoderm. As its name implies, the mesoderm is developed between the original outer cell layer of the blastocyst and the subsequently formed inner cell layer or entoderm. Taking their origin from the growth center at the primitive streak (Fig. 19, B, C), the mesoderipal cells proliferate with astonishing rapidity and establish a third definite and coherent cell layer in the embryonic body (Figs. 20, 21, and 22).
Intra- and Extra-cmbryonic Mesoderm. In its peripheral growth the mesoderm soon extends well beyond the boundaries of the em-, bryonic disk. We may distinguish that part of the mesoderm underlying the embryonic disk as intra-embryonic mesoderm and that part of it which extends peripherally between the trophoblast and the entoderm of the blastocyst as extra-embryonic mesoderm (Fig. 22, A). The distinction is one of convenience in description, but of course purely arbitrary, for there is as yet no line of demarcation between the two areas. It is helpful, however, to realize at the outset that much of the peripheral mesoderm, together with the trophoblast and some of the entoderm, goes into the fabrication of protective and trophic membranes vs^hich later completely envelop the growing embryo. The fact that these membranes are not incorporated in the embryonic body but are discarded at the time of birth is implied in their designation as extra-embryonic membranes.
Fig. 20. Diagrams showing origin, extension, and early differentiation of the mesoderm in a series of pig embryos ranging from about the ninth to the fifteenth day of development, (After Streeter, slightly modified.)
In each figure the embryo is supposed to be viewed in dorsal aspect as a
P'lG. 21 . Diagrams ( X 50) of sections of pig embryos in the early primitive streak stage. (From series in the Carnegie Collection.) A is a longitudinal section; B-E are transverse sections at the levels indicated by the correspondingly lettered lines on the longitudinal section. See also the mesoderm plot, figure 20, D, made from an embryo of about the same age (approximately 10 days).
The Formation of the Notochord
The notochord both phylogenetically and ontogenetically is of great morphological importance. In the most primitive of the vertebrate group it is a well developed transparent object. Except for outlining the embryonic area, only mesodermal structures are represented. The area indicated by heavy horizontal hatching in A, is the thickened part of the embryonic disk from which mesoderm is first proliferated (cf. Fig. 19). Concrescent growth at this region (see arrows in B) gives rise to the primitive streak (cf. shape of horizontally hatched area, A~I). Mesoderm is indicated by crosshatching and the notochord by stippling. The regions in which the coelom has been established by splitting of the lateral mesoderm are indicated by the outlined areas shown in fainter crosshatching. Local thickenings in the mesoderm are indicated by strengthening of the crosshatching. The extra-embryonic mesoderm is omitted in H and I.
Abbreviation: Pericard. thick., thickened mesoderm in future pericardial region.
fibro-cellular cord lying directly ventral to the central nervous system and constituting the chief axial supporting structure of the body. In fishes such as those of the shark family (elasmobranchs) ring-like cartilaginous vertebrae are formed about the notochord. Although somewhat compressed where the vertebrae encircle it, the notochord persists in such forms as a well-defined continuous structure extending throughout the length of the vertebral column. When, in the progress of evolution, cartilaginous vertebrae are replaced by highly developed bony vertebrae the notochord is still more compressed. But even in mammals a minute canal persists for a time in the center of the developing vertebrae marking the position of the notochord. In the early stages of development the notochord of a mammalian embryo is a conspicuous structure, at once a record of evolutionary history, and an advance indication of the location of the vertebral column.
The notochord is established immediately cephalic to the primitive streak. The cells of which it is composed take their origin from the thickened area (Henseri^s node) at the anterior end of the primitive streak. These cells grow out in the form of a rod-shaped mass lying medially in the body. Meanwhile the mesoderm in its peripheral growth has spread out more or less uniformly except in this region.
Fig. 22. Diagrams (X 50) of sections of pig embryos at the beginning of notochord formation. .(From series in the Carnegie Collection.) A is a longitudinal section; B-E are transverse sections at the levels indicated by the correspondingly lettered lines on the longitudinal section. See also the mesoderm plot, figure 20, G, and drawing of an entire embryo, figure 23, made from specimens of the same age (approximately 12 days).
where it has left a temporarily free area (Fig. 20, E, F). It is into this unoccupied space between the ectoderm and entoderm that the notochord grows (Figs. 20, G, H, and 22, A, D).
It will be recalled that the primitive streak is nothing else than an elongation of the original proliferation center of the embryonic disk from which we traced the origin of the mesoderm. The source and manner of origin of the notochord arc therefore similar to that
Fig, 23. Drawing (X 15) of pig embryo about 12 days old. (Carnegie Collection, C181-12.) The ends of the long I thread-like blastocyst (cf. Fig. 18) have been cut off.
described for the previously established mesoderm. The only differences are that the notochord appears somewhat later, and that it grows as a rod-shaped mass of closely packed cells instead of spreading out freely as does the mesoderm (Figs. 20, G-I and 22, A, D).
The Coelom
The mesoderm which spreads out peripherally from the primitive streak is at first a fairly uniform sheet of cells (Fig. 21, A, D). It does not, however, remain long undifferentiated. Sections of slightly older embryos show the lateral portions of the mesoderm splitting into two layers (Fig. 22). The outer layer is called the Somalia mesoderm and the inner layer the sp lanchni c mesoderm. The cavity between somatic and splanchnic mesoderm is the coelom. Because the somatic mesoderm and the ectoderm are closely associated and undergo many foldings in common, it is frequently convenient to designate the two layers together by the term somatopleurc. For the same reasons splanchnic mesoderm and entoderm together arc designated as splanchnopleure.
The splitting of the lateral mesoderm does not occur simultaneously throughout its extent. The earliest indications of the process appear here and there in the more peripheral parts of the mesoderm. The first small local areas of separation give rise to isolated vesicles (Fig. 20, E) which rapidly extend and become confluent to establish the coelom (Figs. 20, E-G, and 22). A definite coelom is thus first established in the extra-embryonic portions of the mesoderm, and is correspondingly designated as the extra-embryonic part of the coelom or, more briefly, exocoelom.
Fig. 24. Drawing (X 15) of pig embryo showing the first appearance of the neural groove. (Carnegie Collection, Cl 60-68.) Compare with mesoderm plot from embryo of same age (approximately 13 days) shown in Fig.
20, H.
As development progresses, the splitting initiated peripherally continues to extend toward the embryo and soon involves the intraembryonic portion of the mesoderm (Fig. 20, G-I). Thus an intraembryonic portion of the coelom is established which is at first directly continuous with the exococlom (Fig. 26, C). Later in development, as the growing embryo is more definitely separated from its surrounding membranes, we shall sec the demarcation between intraand extra-embryonic coelom quite sharply established. The part of the coelom then included within the embryo gives rise to its body cavities (pericardial, pleural, and peritoneal cavities).
F;g. 25. Drawing (X 15) of pig embryo at the time of the appearance of the first somites. (Carnegie Collection, 0190-2 and Cl 96-1.) Compare with the section diagrams in figure 26, and the mesoderm plot in figure 20, I, made from embryos of the same age (about 14 days) .
It is of interest to note in passing that the first part of the intraembryonic coelom to be established is the region where the heart will develop (pericardial region of coelom, Fig. 20, I). This precocious formation of the pericardial coelom presages the early appearance of the cardiovascular system as a whole. Another condition of interest is the extensive development of the extra-embryonic layers which foreshadows an early differentiation of the membranes deriyed from them. The accelerated differentiation of these two systems is a very striking feature of .mammalian development, and would seem to be quite definitely correlated with the paucity of yolk in the mammalian ovum. In the absence of a readily available supply of stored food material, membranes capable of establishing metabolic interchange with the maternal circulation, and a fetal circulation capable of transporting and distributing the food material absorbed through these membranes, are both indispensable factors for the growth of the embryo.
The Mesodermic Somites
The earliest manifestations of metamerism to appear in the body of the mammalian embryo are the segmentally arranged thickenings of the mesoderm called somites. Before the somites themselves are distinguishable there is a clearly marked differentiation of the mesoderm from which they are derived. On either side of the notochord the immediately adjacent mesoderm becomes markedly thickened (Figs. 20, H, and 26, C). In distinction to the lateral mesoderm with which they are continuous, these paired zones of thickened mesoderm constitute the dorsal mesoderm. The narrow region which forms the transition from dorsal to lateral mesoderm is called the intermediate mesoderm, (Fig. 26, C.)
Differentiation of the dorsal mesoderm first becomes apparent about midway between the cephalic and the caudal end of the embryo (Fig. 20, H). Somites are formed from these thickened zones of the mesoderm by a series of transverse divisions brought about by cell rearrangement and resulting in the establishment of block-like masses of more or less radially arranged cells (Figs. 20, I, and 42, A, B). The first pair of somites is formed from the cephalic part of the dorsal mesoderm. As continued growth from the primitive streak region progressively increases the length of the embryo, the first somites formed are carried cephalad in the general expansion of the embryonic body. Keeping pace with the increase in cephalo-caudal elongation, more and more dorsal mesoderm becomes differentiated caudally and new pairs of somites are added behind those previously established.
Caudal Growth and Cephalic Precocity
The fact that at this stage of development the most active growth is taking place from the caudal part of the embryo about the primitive streak, and that the newly formed tissue is being forced thence toward the head, is indicated by other conditions besides the succession in which new somites are added. The size of the primitive streak and its position in the body offer additional evidence concerning the nature of the growth processes going on at this time. In spite of the very rapid cell division which can be seen to occur in it, the primitive streak does not increase in size. Nor does it move cephalad in the growing body as do* the somites. It becomes, on the contrary, a relatively less and less conspicuous structure and retains its original caudal position in the body. The reason for this is the fact that the cells proliferated in the primitive streak region do not remain there but push forth as fast as they are formed. The majority of these new cells are crowded in between the primitive streak and the already established part of the embryo cephalic to the streak. This results in rapid expansion of the body cephalic to the primitive streak. One is very likely, in observing a series of embryos in which the progress of elongation in the cephalic region is so striking, to attribute it entirely to especially active growth in this region itself- In reality it is due rather to rapid growth from behind, which pushes the cephalic region ahead.
The fact that the growth of a young embryo is taking place chiefly from its caudal end has a bearing also on the relative progress of differentiation in different regions of the body. It is a striking fact that the cephalic end of an embryo will always be found precocious in differentiation as compared with the more posterior portion of the embryo. This much-commented-on condition seems but natural when we consider that the head is actually older in development. For the structures posterior to the head are laid down by cells which were proliferated from the growth center at the primitive streak, subsequently to the establishment of the head itsc'lf. Differentiation does ocxur exceedingly rapidly in the head. Were this not so, other regions would pass it in developmental progress. But we cannot, in taking cognizance of this condition, afford to overlook the fact that the head is given a considerable lead at the outset by its earlier establishment.
The Neural Folds
During the period in which the early differentiation of the mesoderm occurs, there have appeared in the embryonic disk indications of the formation of the central nervous system. A broad mesial zone of the ectoderm cephalic to Hensen’s node becomes markedly thickened as compared with the rest of the ectoderm. This thickened part of the ectoderm is called the neural plate (Fig. 26, B). Almost as soon as it is differentiated, the neural plate becomes folded so that its medial portion is depressed and its lateral portions elevated.
The longitudinal groove thus formed in the mid-dorsal surface of the embryo is called the neural groove and the ectodermal folds flanking it are known as the neural folds (Figs. 24, 25, and 26, C). The folding of the neural plate is at this time most pronounced at the level of the first somites, that is, in what is destined to become the hind-brain region of the embryo. Cephalically the broad, well -developed neural plate marks the future fore-brain region. Caudally the neural folds diverge and flatten out. At the primitive streak region even the ectodermal thickening which constitutes the neural plate has disappeared. The central nervous system caudal to the hind-brain region has yet to be laid down even in primordial form. Thus we see in the central nervous system the same conditions of cephalic precocity that were evident in somite formation. The brain region is definitely established before the caudal end of the spinal cord is even foreshadowed by a primordial cell aggregation.
The Embryological Importance of the Germ Layers
In looking back over the development thus far undergone by the embryo, perhaps the most conspicuous thing, at first glance, is the multitude of cells formed from the single fertilized egg cell by repeated mitoses. Of more significance, however, is the fact that even during the early phases of rapid proliferation the cells thus formed do not remain as an unorganized mass. Almost at once they become definitely arranged as a hollow sphere which is called the blastocyst. Scarcely is the blastocyst established as a single-walled vesicle when, from the knot of cells which constitute the inner cell mass, certain cells migrate out and become arranged as a second layer inside the first. This second layer, because of its position inside the original layer, is called the entoderm. Shortly a thhd cell layer makes its appearance between the first two, being called, appropriately enough, the mesoderm. That part of the original wall of the blastocyst which still constitutes its outer covering after the entoderm and mesoderm have been established is now properly called the ectoderm. These three cell layers are spoken of as the germ layers of the embryo.
The germ layers are of interest to the embryologist from several angles. The very simple organization of the embryo when it consists of first a single, then two, and finally three primary structural layers is reminiscent of ancestral adult conditions carrying far back into the invertebrate series. Frdm the standpoint of probable ontogenetic recapitulations of remote phylogenetic history several facts are quite suggestive. The nervous system of the vertebrate embryo arises from
Fig. 27. Chart showing the derivation of the v?»rious structures of the body by progressive differentiation and divergent specialization. Note especially how the origin of all the organs can be traced back to the three primary germ layers, ectoderm, entoderm, and mesoderm.
Ectoderm
the layer through which a primitive organism which has not as yet evolved a central nervous system is in touch with its environment. The lining of the vertebrate digestive tube is formed from the entoderm — the layer which in very primitive forms lines a gastrula-like enteric cavity. The chief vertebrate skeletal and circulatory structures are derived from the mesoderm- — the layer which in small, lowly organized forms is relatively inconspicuous but which constitutes a progressively greater proportion of the total bulk of animals as they increase in size and complexity and consequently need more elaborate supporting and transporting systems.
Interesting as are the possibilities of interpreting the germ layers from the standpoint of their phylogenetic significance, our chief concern with them centers about the part they play in the development of the individual. Their establishment marks the first segregation of cell groups which are clearly distinct by reason of their definite positional relations to each other, and within the embryo. This positional relationship, moreover, is fundamentally the same for the germ layers of all vertebrate embryos— a fact which speaks forcefully of the common ancestry of all the members of this great group.
Of more importance still is the fact that the different germ layers contain cells with different developmental potentialities. As development progresses, cell groups of given potentialities are, so to speak, sorted out of the germ layers, perhaps by being folded off from the parent layer, perhaps by migrating out as individual cells and later becoming re-aggregated elsewhere. The story of the embryological origin of the various parts of the body is the history of the growth, subdivision, and differentiation of the germ layers. From cell groups thus derived the organs with which we are familiar in the adult gradually take shape.
The idea of repeated regrouping and progressive differentiation and specialization is expressed graphically iti figure 27. This chart at the present stage of our study will serve as a means of pointing out in a general way where the early processes we have been dealing with are leading. As we follow the phenomena of development farther we shall find each natural division of the subject centers more or less sharply on a certain branch of this genealogiccil tree of the germ layers.
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