The Development of the Individual (Embryology)
The Starting Point
No animal or plant is an orphan in the sense that it has no parents. In protozoans produced by fission the parent perishes by division into progeny. The spontaneous origin, even of the most minute organisms, at least under present conditions on this globe, has been effectually disproved. Modern control of bacterial diseases, together with th,e incalculable boon of aseptic surgery and antiseptic practice, depends upon the clear understanding of this fact.
A new organism may be introduced into the brotherhood of living things by one parent or by two, but no animal or plant comes into the world of today unsponsored by preceding life.
When there is only one parent the new individual is said to arise by asexual reproduction. The method of sexual reproduction, however, that involves a double source, is by far the commoner way for higher animals and plants to begin their separate existence.
The starting point of a sexually produced individual is a zygote, or fertilized egg, which is a combination of two parental gametes, or mature sex cells. It is the purpose of this chapter to trace some of the more important episodes in the “miraculous pageant of transformations” that take place between the setting up of the zygote and the establishment of the adult organism. A fascinating part of biology is the particular province of Embryology, some familiarity with which is essential to the understanding of the structure and functions of adult animals and plants.
Ever since Aristotle described what he saw when he opened hen’s eggs at various stages of incubation, embryology has been occupied with recording what changes take place during development, with the result that a considerable body of detailed information has been accumulated. In recent years embryologists have not been content simply to find out what happens in a transforming embryo, and how the changes occur, but have sought more and more to explain why such changes take place. This attempt has led to Experimental Embryology, an aspect of biological science with alluring vistas that is claiming much attention today.
Why does the power to regenerate lost parts decrease as we go up the vertebrate scale? Why does the orderly sequence of development sometimes become upset so that a monstrous organism results? Why do groups of cells sometimes go on a rampage and form unorganized cancerous growth that the animal cannot control?
The Necessary Partners
Differentiation of the Germ Cells
Two germ cells, sperm and egg in animals, are necessary partners in the enterprise of a fertilized egg. The fact that different terms, namely, pollen grains and ovules, are commonly employed to designate the reproductive units in sexual plants does not indicate any essential difference in the germ cells of plants and animals. A pollen grain is a spore which produces an organism, the pollen tube, from which a fertilizing element or cell, homologous with the animal sperm, passes to the ovule.
Fertilization is the union of two diverse germ cells, and consequently provision has to be made for getting them together.
The egg, which is ordinarily loaded with first-aid nutriment for the future organism during the critical early stages of its development, tends to become relatively heavy and stationary, thus throwing upon the sperm the responsibility of doing the traveling in the necessary process of getting together. The egg does not meet the sperm half way. The sperm has to travel the entire distance. This circumstance has brought about a high degree of morphological difference in the germ cells of the two sexes.
Kinds of Sperm
The result of the physiological necessity of a union of germ cells to effect fertilization is that the sperm cells of animals, and the male cell in the pollen grains of plants, become specialized into structures adapted particularly for locomotion. In animals this involves a fluid medium in which to travel. In plants the traveling male germ cell within the pollen grain is more often adapted for transport through the air by the wind or through the agency of insects.
A typical sperm cell adapted for locomotion along a fluid highway is pictured in Figure 92h. The human sperm cell, according to Waddington, can travel at the rate of about an inch in three minutes.
The differentiated head of the sperm, which is principally made up of the nucleus, carries the chromosomes that are freighted with the hereditary determiners, while the middle piece and the locomotor tail represent transformed cytoplasm, modified for particular uses. An animal sperm is thus adapted for sculling forward through a fluid medium by means of the vibratile tail. Animals never employ aerial routes for this purpose as do plants.
Among vertebrates, bony fishes and anurans usually broadcast their eggs and sperm in water, and the sperm cells travel in this medium to reach the egg. Among land forms like reptiles, birds, and mammals, internal fertilization occurs, so that the sperm travel up the oviduct to the egg in a secreted fluid medium that serves the same purpose as water in the case of aquatic animals. Copulation, which occurs in all land animals, is simply a device for insuring placement of locomotor sperm cells in a suitable highway leading to the waiting egg.
An exception to the almost universal type of sperm with a locomotor tail is found among certain worms, Ascaris fpr example, and in crabs, where an amoeboid or angular form is assumed by the sperm cell. The approach to the egg in this instance is accomplished by much slower movements, the creeping sperm being in contact with solid objects.
Kinds of Eggs
The eggs of animals differ specifically with reference to the load of nutritive yolk which they carry. Curiously those with a minimum amount of stored food are found at the two extremes of the chordate scale, namely, the eggs of amphioxus and of mammals. The eggs of amphioxus probably represent a primitive condition in the matter of the acquisition of yolk. The poverty of yolk in the small eggs of mammals has doubtless come about through a different chain of causes, correlated with the fact that not much stored food is required in the eggs of this group, since they early become implanted like parasites in the uterine wall of the mother from whom they derive their necessary nutritive start in life. As a consequence of the scarcity of yolk, the mammalian egg is remarkably small, that of man measuring only about 1/125 of an inch in diameter (Fig. 110). As the yolk is scattered equally through these eggs, they are described as isolecithal (iso, equal; lecithin, yolk).
In cyclostomes, fishes, and amphibians, the abundant lifeless yolk is massed in the lower or “vegetal” half of the egg while the nucleus and most of the living cytoplasm, constituting the embryogenic or “animal” pole, appears on the upper side. Such eggs are therefore telolecithal (“end-yolked”).
So much yolk is present in the telolecithal eggs of reptiles and birds that the nucleus of the egg cell, together with its tiny halo of active cytoplasm, forms only a small area, or germinal disc, at the animal pole. When unhindered the heavy yolk invariably rotates so that the germinal disc comes to lie uppermost.
In addition, birds have a reserve food supply of nutritive albumen or “white,” packed around the egg within the protective shell.
If one stretches the point to include such accessory food material, the bird’s egg must be regarded as the largest kind of all animal cells. The egg of the wingless Apteryx of the Antipodes weighs nearly one fourth as much as the entire bird. An ostrich’s egg is equivalent in bulk to about a dozen hen’s eggs, while that of the gigantic “moa,” now extinct, had twelve times the content of an ostrich’s egg and might, therefore, easily hold the palm as being the largest animal “cell” ever known.
Development of a Chordate
The manner of development of an egg is dependent upon the amount of inert yolk that is present. In amphioxus, where there is very little yolk to hinder the process of orderly cell division, the entire egg mass is equally involved in cell formation, and early development is less complicated than in most vertebrates. Before considering specific points for several representative chordates, let us first examine the fundamental plan of chordate development.
Cleavage (Segmentation of the egg)
After the union of the sperm and egg, the first of a long series of processes that transform the fertilized egg into an adult individual occurs. This first process, called cleavage, consists of a rapid succession of mitoses in which the initial cell becomes divided in turn into two, four, eight, and so on (Fig. 111), until a small mass of cells results, without appreciable increase in total weight over that of the fertilized egg. Each one of these small cells contains a complete double set of chromosomes, bearing hereditary potentialities from two parents, thus duplicating the original complement in the fertilized egg.
The result of these rapid preliminary cleavage divisions is the breaking up of the original cell into many separate working units. A very fundamental principle underlying differentiation is division of labor, and this is facilitated when there are different nuclear centers present for the initiation of different cellular enterprises.
As the small cells, of essentially uniform size, become more numerous, they arrange themselves in the form of a hollow sphere, or blastula, the cavity within being known as the segmentation cavity and the individual cells, blastomeres.
The cells at one pole of the hollow sphere divide oftener and become more numerous and crowded than at the other pole. Since they remain in contact with each other without changing their relative positions to any great extent, they tend to form a continuous layer that is more than sufficient to make up the original surface of the sphere in this region. They are, therefore, forced to find more standing room which is accomplished by their pushing into the segmentation cavity, with the result that a double cup, or gastrula, is formed (Fig. 112).
The outer layer of this cup is the ectoderm, the inner layer the endoderm (or “entoderm” of some authors). The new cavity within the cup is termed the archenteron, or primitive digestive cavity, and its opening to the exterior, the blastopore. The latter is at the posterior end of the embryo.
The inpushing (invagination) of the endoderm continues until the segmentation cavity is almost obliterated. Meanwhile the addition of new material in a growth region near the posterior end of the gastrula, around the blastopore, brings about an elongation of the embryo and a marked decrease in the size of the blastopore. As this elongation is more rapid ventrally than elsewhere, the reduced blastopore is moved to a somewhat dorsal position instead of remaining strictly terminal. This more rapid elongation of the ventral side of the gastrula is a good illustration of the far-reaching principle of unequal growth in the process of differentiation. In fact “unequal growth,” that is, unequal in quantity or rate, lies at the very foundation of many processes of morphogenesis, constituting much of the subject matter of embryology.
Formation of the Nervous System
One of the earliest organ systems to appear is the nervous system. Along the dorsal side of the gastrula, ectoderm cells begin to multiply at a greater rate than in the surrounding region, thus forming a thickened area, the neural plate. Further increased mitotic activity in these cells soon results in the formation of a groove along the middle of the neural plate, as the cells in the mid-dorsal region push down into the underlying segmentation cavity and those on either side simultaneously are folded up into longitudinal ridges, neural folds (Fig. 113). The furrow between the ridges is the neural groove, the walls of which are the forerunner of the central nervous system. The neural folds continue to approach one another until they meet above the invaginated groove to form the neural tube which soon splits off from the surface ectoderm. This process, beginning in the head region, proceeds posteriorly. Consequently when the groove has been completely closed over near its anterior end, the neural folds may be merely beginning to appear in the posterior part of the embryo. The extreme anterior end of the groove remains open for a time, however, as the neuropore which leads into the cavity of the neural tube, the neurocoele.
Gradually the neural folds extend farther and farther toward the posterior end of the embryo until they reach points on either side of the tiny blastopore which, as a result, then lies in the bottom of the neural groove. As soon as the two folds have extended beyond the blastopore they turn toward the mid-line where they meet (Fig. 114). As at other levels, the folds in this posterior region eventually meet over the top of the groove and complete the formation of a tube. Because the neuropore was on the floor of the groove, there is now a passageway (neurenteric canal) between the neurocoele and the primitive gut cavity. This connection is later obliterated, leaving the two systems completely separate in this region.
Formation of Notochord and Mesoderm
Concurrently with the appearance of the central nervous system, three groups of cells grow into the dorsal part of the segmentation cavity, either from the dorsal endoderm or from the zone of proliferation just in front of the blastopore. One of these groups, lying in the mid-line, forms an unpaired strand of cells which later separates from the parent tissue to become the notochord. The other two groups of cells, one on each side of the notochord, are the paired beginnings of the third germ layer, the mesoderm (Fig. 113).
Through rapid cell proliferation these mesodermal sheets spread laterally and ventrally around the endoderm. Meanwhile each sheet has split into an outer somatic layer, adjacent to the ectoderm, and an inner splanchnic layer, next to the endoderm. The new cavity thus formed, the coelom, lying wholly within the mesoderm, is a single continuous space on each side of the body.
Differentiation of the Mesoderm
As the mesodermal sheets spread and split, each undergoes differentiation into three regions, namely: (1) a dorsal epimere, near the neural tube; (2) a small mesomere, and (3) a ventral hypomere, enclosing a large part of the coelomic cavity (Fig. 115). The epimere, beginning first near the anterior end, becomes gradually divided transversely into parts known as somites, which soon become separated from the mesomere. Only weak and temporary segmentation ever appears in the mesomere, while the hypomere never shows evidence of segmentation or of separation from the mesomere.
The thin-walled hypomeres of the two sides grow toward the mid-line, both dorsal and ventral to the archenteron, until they come in contact with one another to form the two-layered dorsal and ventral mesenteries (Fig. 116). Although the dorsal mesentery persists in the adult animal, nearly all of the ventral mesentery soon disappears so that the two coelomic cavities become continuous. Between the thin, approximated walls of the dorsal mesentery, blood vessels and nerves extend to and from the digestive tube.
Emigration of the Mesenchyme
From the splanchnic part of each hypomere and both portions of each epimere, cells, collectively known as mesenchyme, migrate into the segmentation cavity. By amoeboid movement, they may wander anywhere within this cavity. Many cells from the median part of each epimere mass alongside the notochord and nerve cord to form a sclerotome (“skeletal segment” ), while most of those from the lateral part of the epimere gather just beneath the ectoderm as the dermatome (“dermal segment”). The portion of the epimere remaining after these mesenchymal cells have been given off is known as the myotome (“muscle segment”). See Fig. 116.
Some of the mesenchymal cells from the hypomere group about the endoderm to form the smooth muscles, blood vessels and connective tissue of the wall of the digestive tract. Other mesenchymal cells, from both hypomere and epimere, migrate throughout the segmentation cavity to form, at appropriate places, connective tissue, cartilage, bone, smooth muscles, blood cells and blood vessels.
After mesenchymal migration has begun, the dermatomes and thin portions of the myotomes grow ventrally into the region between the somatic mesoderm of the hypomere and the ectoderm. With the filling in of dermal cells in the mid-line, both dorsally and ventrally, a continuous sheet of material, which differentiates into the derma, is laid down. Although the myotomes also grow to the mid-line, dorsal to the nerve cord and ventral to the body cavity, the muscles of the two sides of the body never fuse, but remain separated by a thin partition of connective tissue. The ventral, thin portions of the myotomes give rise to the several thin sheets of voluntary muscle tissue of the ventral and lateral parts of the body wall.
Assembling of the Digestive Tube
With the closing over of the blastopore, no direct opening into the archenteron remains. Later a digestive tube, with an inlet at one end and an outlet at the other, is established. The larger part of this food tube is made up of the archenteron, lined with endoderm, already in use. The original archenteron is supplemented at either end by ectodermal invaginations that come in contact with it and finally break through, thus forming a continuous canal through the body of the embryo. The inpushing of the ectoderm at the anterior end, which marks the region of the future mouth, is called the stomodaeum, while the corresponding invagination at the posterior end, that forms the anal exit of the food tube, is called the proctodaeum (Fig. 117). Thus it comes about that food passing through the alimentary tract first rubs against walls of ectodermal origin, then follows along the major distance in contact with endodermal walls where much of it is absorbed, and finally the residue passes out through ectodermal walls.
The Fate of the Germ Layers
With gastrulation and differentiation of the primitive germ layers there begins to be an increase in the size of the embryo, or growth, accompanied by diversification and establishment of the organs and systems that constitute the mechanisms of the adult animal. It is the task of Organology, or Descriptive Embryology, to follow out the changes that take place. Obviously within the confines of a brief introductory chapter it is necessary to avoid many alluring side alleys that entice one from the main highway, and to be content with a brief resume of the structures formed by the several regions just described (Fig. 118).
The ectoderm gives rise to: (1) the nervous system, including the brain, spinal cord, nerves, and receptor endings; (2) the lining epithelium of the mouth and nasal cavities (from stomodaeum) and of the last part of the rectum (from proctodaeum); and (3) the epidermis and all of its derivatives, including feathers, hairs, nails, claws, scales (except in fishes), integumentary glands, enamel of the teeth, and lens of the eye.
The endoderm forms not only the lining of almost the entire digestive tract but also the epithelial layer in the tubules of such outpocketings of the tract as the lungs, liver, and pancreas.
Of the mesodermal regions the epimeres give rise to: (1) mesenchymal sclerotomes which develop into the vertebral column; (2) mesenchymal dermatomes which form most of the dermal part of the integument; (3) myotomes from which nearly all of the voluntary muscles arise; and (4) other mesenchymal cells which contribute to the formation of skeletal, dermal, and circulatory structures as well as smooth muscles.
The mesomeres are the source of the excretory system and the gonads which harbor the germ cells.
The hypomeres, in addition to forming mesenteries and linings of the body cavities, also give rise to some of the voluntary muscles of the head and neck regions and to part of the mesenchymal aggregate from which develop portions of the skeletal, dermal, and circulatory organs, and the smooth muscles.
The question of how these structures arise from their embryonic antecedents awaits our attention in later chapters.
Early Development of Telolecital Eggs
The preceding section has concerned itself with a simplified plan of development which might be followed by an isolecital egg of a lower vertebrate. In many respects this plan also applies to the development of telolecithal eggs but it is modified, particularly in early stages, by the presence of yolk.
When the yolk is disposed polar fashion, as in an amphibian’s egg, the mitoses at the embryogenic animal pole, in the neighborhood of the original nucleus, go forward at an accelerated rate, while cell division is retarded at the opposite vegetal pole where the inert yolk is particularly in evidence (Fig. 119). A blastula is evidently formed but the segmentation cavity within the hollow sphere is eccentric, its walls being of very unequal thickness, because the blastomeres at the animal pole are considerably smaller and more active than those at the opposite or vegetal pole (Fig. 120).
In reptiles and birds the results of segmentation are still further modified by the relatively enormous amount of yolk present. The nucleus of the fertilized egg undergoes the usual mitoses, but the new cell boundaries fail to be extended so as to include the great sphere of yolk material. The result is a patch or disc of crowded blastomeres of unequal size at the animal pole, the larger cells with incomplete boundaries being at the periphery (Fig. 121). Although earlier even the central cells were not separated from the yolk below, they have by this time split off leaving a space, the segmentation cavity, between them and the yolk (Fig. 122).
In the amphibian egg, in which the segmentation cavity has walls of unequal thickness, a slight invagination to form a groove appears on one side at the point where the upper thin wall passes over into the lower thick wall of the vegetal pole. The small cells of the upper lip (on the animal pole side) of the groove now proliferate rapidly, and the lip begins to grow down over the large yolk-filled cells (Fig. 123). The outer layer of the lip is ectoderm, the inner layer endoderm, and the cavity between endoderm and yolk cells is the archenteron. Aided by some growth of the small cells over the large ones all around the equator of the blastula, the dorsal lip eventually covers all but a small yolk plug, composed of the large cells. This plug occupies what is actually the blastopore. Meanwhile the yolk cells themselves have been gradually shifting away from the small archenteron into the segmentation cavity, with the result that the archenteron has increased considerably in size, mainly at the expense of the segmentation cavity. As this gastrulation is taking place, the notochord and two mesodermal sheets are arising from the dorsal endodermal cells and the zone of proliferation in the region of the dorsal lip of the blastopore. The neural tube has also been developing in a manner similar to that described above, though differing in some details. In most respects further development of the various embryonic tissues follows the general chordate plan.
In reptiles and birds, a crescentic fold of tissue forms on one side at the edge of the disc of blastomeres, sending cells underneath between the yolk and the disc (Fig. 124). The proliferation of cells, becoming most marked in the mid-part of the fold, soon becomes evident in the disc cells in a narrow line extending forward from the edge toward the center of the disc until there is a longitudinal streak of cells, primitive streak, in what is really the posterior part of the now elongating disc (Figs. 125 and 126). Cells arising in great numbers from the primitive streak are added to those derived from the infolding to form an endodermal layer between the yolk and those disc cells which remain on the surface as the ectoderm. The space between endoderm and yolk is a greatly modified archenteron, the floor of which is the non-cellular yolk mass.
Further cell proliferation from the primitive streak region gives rise to three parts: an unpaired cord of cells, the notochord, extending forward from the streak; paired sheets of mesoderm, growing laterally from the streak and spreading forward alongside the notochord. Meanwhile the ectoderm above the notochord, in front of the streak, is forming the neural plate and folds which develop into the central nervous system as in most chordates (Fig. 127).
In this manner the original patch of blastomeres on top of the big yolk mass have given rise to the nervous system, notochord, and the three germ layers. The cells on the outside have become the ectoderm, those underneath next to the yolk, the endoderm, while the mesodermal cells are proliferating between them. These pioneer cells and their descendants then set out to spread over and enwrap the entire yolk. As these layers spread out the mesoderm differentiates into epimere, mesomere, and hypomere, while mesenchyme cells appear and organize various parts in the same general manner described for most chordates.
Eventually the embryonic tissues grow entirely around and enclose the yolk in a yolk sac, as in a bag (Fig. 128). At the same time the embryo proper becomes raised up and separated from the yolk mass except for a slender yolk stalk, through which the cavity of the gut is continuous with that of the sac. Despite this continuity, the transfer of nutritive material from the sac to the embryo is apparently entirely through vitelline blood vessels which spread over the yolk sac with the advancing embryonic layers.
Once the three primary germ layers become established, as described above, the further development of a reptile or bird follows the general chordate plan.
Early Development of Mammals
The mammalian egg does not behave in segmentation like amphioxus, which it resembles in its small supply of yolk. The reason for the difference in development is probably that mammals have inherited developmental traditions from a series of ancestors which amphioxus never had. In mammalian cleavage the entire egg is equally divided into blastomeres but without the regularity characteristic of amphioxus, so that, instead of a hollow blastula, an irregular solid mass of cells is formed.
Later the outer blastomeres of the germinal mass make a somewhat distinctive layer, enclosing more spherical central cells (Figs. 129 and 130).
Fluid collects within this mass and a hollow sphere results. The outer enveloping layer of cells, the trophoblast, comes into intimate contact with the inner wall of the uterus at the point where the developing embryo is implanted. Within the sphere is an eccentrically located inner cell mass, which is destined to give rise to all the cells that are to take part directly in the formation of the embryo.
Some of the cells of the eccentric inner cell mass migrate and spread out to form a layer, the endoderm, lining the fluid-filled cavity (Fig. 131). This cavity now corresponds to the combined archenteron and cavity of the yolk sac, only here the entire space is filled with fluid, while in reptiles and birds there is a small liquid-filled archenteron and a large yolk-filled sac. The remaining cells of the inner mass, now the ectoderm, next spread out into a flat “embryonic plate” which corresponds to the ectodermal disc of the reptile or bird. The portion of the endodermal layer which is immediately beneath the “plate” belongs to the embryo proper, while the rest of this inner layer represents the yolk sac. Thus the parts present at this time may be homologized with those of the gastrula of a bird or reptile. Further development of the mammalian embryo follows, in many ways, the ancestral pattern cut out for it by the reptiles, including the appearance of a primitive streak, in the embryonic plate, from which cells grow out to produce notochord and mesoderm (Fig. 132).
From these few brief statements concerning the early stages in the development of various vertebrates it is clear that gastrulation and the formation of mesoderm and notochord vary considerably among the several vertebrate classes. It should be emphasized, however, that once the three primary germ layers become established they give rise to the principal organ systems with great uniformity, as shown in Figure 118.
The Major Cavities
A coelomic cavity usually develops on each side of the body in the manner we have described, by the splitting of the mesodermal sheet. Hence it is a schizocoele (schizo, split). In amphioxus, however, the anteriormost mesoderm arises as a series of outpocketings, or pouches, from the dorsolateral regions of the archenteron (Fig. 133). After the mesodermal pouches separate from the archenteron the cavities of all those on each side of the body combine to form a continuous coelom, usually known as an enterocoele because of its origin from the archenteron. This method of mesoderm formation does not occur in the vertebrates, with the probable exception of some amphibia, but is typical of hemichords and echinoderms. This formation of enterocoeles in echinoderms and some chordates as well as the posterior position of the blastopore in echinoderms and all chordates indicates that these two phyla may be closely related.
Mesenteries and other parts of the mesoderm arise in much the same manner whatever the method of formation of their germ layer may be. In either case the paired coelomic cavities, confined mainly to the hypomeric region, unite into a single cavity upon the disappearance of most of the ventral mesentery. In birds and mammals the embryonic coelomic cavity becomes divided into three different sorts of spaces, namely: the pericardial cavity enclosing the heart, the two pleural cavities surrounding the lungs and the peritoneal cavity housing chiefly the major part of the digestive tract and the urogenital organs (Fig. 134).
As the first evolutionary step in lower vertebrates, the coelom becomes divided by a double transverse mesodermal wall, the transverse septum, into a small anterior pericardial cavity and a large posterior peritoneal or abdominal cavity. In some fishes (e.g. Squalus) this septum is not quite complete, so that a communication between the two cavities persists throughout life in the form of the so-called pericardio-peritoneal canal lying along the ventral side of the esophagus.
With the appearance of lungs, which grow back into the anterior part of the abdominal cavity, the name pleuroperitoneal cavity is more properly applied to this region of amphibia and reptiles. Usually the two lungs are in more or less individual forward extensions of the main cavity. In most of these animals the heart shifts posteriorly to lie ventral to the anterior part of the abdominal cavity, with the result that the ventral part of the transverse septum is pushed posteriorly while the dorsal part keeps its more anterior attachment.
In birds and mammals a new partition develops, extending from the ventral part of the transverse septum to the dorsal body wall. Although membranous in birds, it is invaded by myotomic muscle tissue in mammals to become the muscular diaphragm. In mammals the cavity anterior to this new partition is known as the thoracic cavity, while the one posterior to it becomes once more the peritoneal cavity, for the lungs are no longer included here. The thoracic cavity contains a pericardial cavity, ventrally in the center, and two pleural cavities, one on each side, lateral and dorsal to the heart.
In connection with this discussion it should be borne in mind that no organs are actually in the coelom but all, instead, are in the segmentation cavity. An organ may push into the coelom but it always carries ahead of it a fold of the peritoneal wall (Fig. 135).
When an egg is fertilized the entering sperm is something added to it from without, starting a series of internal changes that finally result in the adult body. Other outside agents, like a pinprick or contact with certain chemicals, may also, in certain cases, start up the cleavage of an egg as if it had been fertilized by a sperm. If the sequence of internal events is interrupted, or fails to occur in the nick of time, the whole subsequent procedure is upset. What is the internal mechanism that regulates this marvelous performance, once it is initiated?
It has been discovered that it is possible to transplant a bit of one embryo to an unnatural position in another embryo, and that the transplant carries out its original structural tradition even in the unnatural surroundings of its host’s body. For example, the embryonic bud of a tadpole destined normally to grow into a leg, when transplanted to the back of another tadpole, will still carry out its original design and form a leg, even in so bizarre a location.
If one blastomere of a frog’s egg, when it is in the two-cell stage of cleavage, is killed by stabbing the nucleus with a hot needle, the other blastomere will carry on and develop a hemi-embryo which may eventually restore the missing half embryo and complete the pattern of the entire embryo (Fig. 136).
The ability to perform such a recovery or to develop an organ from an extirpated embryonic bud lasts for only a critical brief period. Once this time is past, if the sequence of normal events is interrupted, the internal mechanism is unable to carry out the original structural design. The particular region of a developing organism that possesses this magical power of directing internal operations is called an organization center.
One of the earliest organization centers is around the dorsal lip of the blastopore where the mesoderm is organized. This primary center is succeeded by other formative centers, secondary, tertiary, and so on, each of which is dependent upon the successful operation of preceding centers. The discovery of such formative centers by Spemann, Harrison, and others through experiments upon developing embryos, is a promising beginning towards solving the problem of why the architectural plan of a particular species is carried out successfully in the innumerable individuals which grow to maturity.
Soma and the Germ-Line
In the long series of mitoses that follow the initial fertilized egg, there comes a time when the two daughter cells resulting from some particular cell division are no longer identical twins in their differentiation. They may still have the same kind of chromosomal equipment as the result of a preceding mitosis, and may be indistinguishable in appearance, but, as their future behavior shows, they have come to a fundamental parting of the ways, for one of the pair goes on as one of the ancestors of all the myriad cells that differentiate into various tissues and organs to form the growing individual (soma), while the other becomes the ancestor of all eggs or sperm (germ-line), and so is charged with the necessary business of reproducing the species.
There are pronounced differences in these two streams of differentiating cells. The soma becomes the conspicuous thing which is known as the animal or plant body, and is biologically the guardian of the inconspicuous and less commonly known germ-line. The soma is mortal, for after a time it inevitably breaks down and dies either a natural or a violent death. The cells of the germ-line, on the other hand, although they may perish with the dying soma, are potentially immortal, since they form the only biological bridge in vertebrate animals across which the spark of life may be borne from one generation to another.
It is quite possible to go backward in imagination step by step without a break in the life-line of living cells, from any particular individual cell of an adult organism to the fertilized egg from which it came, and to see how the material in that fertilized egg was in turn a part of the unbroken series of cells of the germ-line that were housed in preceding generations of somas, and so on to the very remotest ancestral source.
The soma within limits can maintain and repair itself. The germ-line can not only do that but it can also give rise to new somas (Fig. 137). This is its mission, to reproduce new individual organisms, while it is the business of the soma, or the individual body, to nourish, protect, transport, and unite germ-lines. Otherwise inevitable death ends all.
The Succession of Generations
The science that deals with the germ-line is Genetics. The resemblance everywhere so apparent between individuals of successive generations of a species has its explanation in the fact that both parent and offspring are somatic expressions of the same germ-line. That is why “pigs is pigs,” and chickens hatch out of hen’s eggs. The laws of heredity are fundamentally concerned, therefore, with the behavior of the germ-line and its expression in the soma.
There are various ways to get at the matter. In the past the approach to the problems of heredity has been made usually by comparing points of likeness and difference in individuals of succeeding generations of a species. This somatic method is facilitated by the experimental breeding of plants and animals. During the last forty years great advance has been made in such breeding by resort to the fundamental principles known as “Mendelism.”
Another line of approach is the direct study of the germ-line, which has given rise to an increasing army of biological specialists, who are concerned with the intimate behavior of hereditary units, or genes, located in chromosomes, particularly those of the germ cells.
To these investigators we are indebted for an expanding body of knowledge about spermatogenesis and oogenesis in animals and plants, as well as for the facts and laws which concern germplasmal origins.