The Evolution of Man, V.1. - Ernst Haeckel (learn to read activity book TXT) 📗

rightly observes: firstly, formation of the entoderm by cleavage at the centre and further growth at the edge; secondly, invagination. In the monotremes more primitive conditions have been retained better than in the reptiles and birds. In the latter, before the commencement of the gastrula-folding, we have, at least at the periphery, a two-layered embryo forming from the cleavage. But in the monotremes the formation of the cenogenetic entoderm does not precede the invagination; hence in this case the construction of the germinal layers is less modified than in the other amniota.

The marsupials, a second sub-class, come next to the oviparous monotremes, the oldest of the mammals. But as in their case the food-yelk is already atrophied, and the little ovum develops within the mother's body, the partial cleavage has been reconverted into total. One section of the marsupials still show points of agreement with the monotremes, while another section of them, according to the splendid investigations of Selenka, form a connecting-link between these and the placentals.

(FIGURE 1.64. Blastula of the opossum (Didelphys). (From Selenka.) a animal pole of the blastula, v vegetal pole, en mother-cell of the entoderm, ex ectodermic cells, s spermia, ib unnucleated yelk-balls (remainder of the food-yelk), p albumin membrane.)

The fertilised ovum of the opossum (Didelphys) divides, according to Selenka, first into two, then four, then eight equal cells; hence the segmentation is at first equal or homogeneous. But in the course of the cleavage a larger cell, distinguished by its less clear plasm and its containing more yelk-granules (the mother cell of the entoderm, Figure 1.64 en), separates from the others; the latter multiply more rapidly than the former. As, further, a quantity of fluid gathers in the morula, we get a round blastula, the wall of which is of varying thickness, like that of the amphioxus (Figure 1.38 E) and the amphibia (Figure 1.45). The upper or animal hemisphere is formed of a large number of small cells; the lower or vegetal hemisphere of a small number of large cells. One of the latter, distinguished by its size (Figure 1.64 en), lies at the vegetal pole of the blastula-axis, at the point where the primitive mouth afterwards appears. This is the mother-cell of the entoderm; it now begins to multiply by cleavage, and the daughter-cells (Figure 1.65 i) spread out from this spot over the inner surface of the blastula, though at first only over the vegetal hemisphere. The less clear entodermic cells (i) are distinguished at first by their rounder shape and darker nuclei from the higher, clearer, and longer entodermic cells (e), afterwards both are greatly flattened, the inner blastodermic cells more than the outer.

(FIGURE 1.65. Blastula of the opossum (Didelphys) at the beginning of gastrulation. (From Selenka.) e ectoderm, i entoderm; a animal pole, u primitive mouth at the vegetal pole, f segmentation-cavity, d unnucleated yelk-balls (relics of the reduced food-yelk), c nucleated curd (without yelk-granules).

FIGURE 1.66. Oval gastrula of the opossum (Didelphys), about eight hours old. (From Selenka) (external view).)

The unnucleated yelk-balls and curd (Figure 1.65 d) that we find in the fluid of the blastula in these marsupials are very remarkable; they are the relics of the atrophied food-yelk, which was developed in their ancestors, the monotremes, and in the reptiles.

In the further course of the gastrulation of the opossum the oval shape of the gastrula (Figure 1.66) gradually changes into globular, a larger quantity of fluid accumulating in the vesicle. At the same time, the entoderm spreads further and further over the inner surface of the ectoderm (e). A globular vesicle is formed, the wall of which consists of two thin simple strata of cells; the cells of the outer germinal layer are rounder, and those of the inner layer flatter. In the region of the primitive mouth (p) the cells are less flattened, and multiply briskly. From this point--from the hind (ventral) lip of the primitive mouth, which extends in a central cleft, the primitive groove--the construction of the mesoderm proceeds.

Gastrulation is still more modified and curtailed cenogenetically in the placentals than in the marsupials. It was first accurately known to us by the distinguished investigations of Edward Van Beneden in 1875, the first object of study being the ovum of the rabbit. But as man also belongs to this sub-class, and as his as yet unstudied gastrulation cannot be materially different from that of the other placentals, it merits the closest attention. We have, in the first place, the peculiar feature that the two first segmentation-cells that proceed from the cleavage of the fertilised ovum (Figure 1.68) are of different sizes and natures; the difference is sometimes greater, sometimes less (Figure 1.69). One of these first daughter-cells of the ovum is a little larger, clearer, and more transparent than the other. Further, the smaller cell takes a colour in carmine, osmium, etc., more strongly than the larger. By repeated cleavage of it a morula is formed, and from this a blastula, which changes in a very characteristic way into the greatly modified gastrula. When the number of the segmentation-cells in the mammal embryo has reached ninety-six (in the rabbit, about seventy hours after impregnation) the foetus assumes a form very like the archigastrula (Figure 1.72). The spherical embryo consists of a central mass of thirty-two soft, round cells with dark nuclei, which are flattened into polygonal shape by mutual pressure, and colour dark-brown with osmic acid (Figure 1.72 i). This dark central group of cells is surrounded by a lighter spherical membrane, consisting of sixty-four cube-shaped, small, and fine-grained cells which lie close together in a single stratum, and only colour slightly in osmic acid (Figure 1.72 e). The authors who regard this embryonic form as the primary gastrula of the placental conceive the outer layer as the ectoderm and the inner as the entoderm. The entodermic membrane is only interrupted at one spot, one, two, or three of the ectodermic cells being loose there. These form the yelk-stopper, and fill up the mouth of the gastrula (a). The central primitive gut-cavity (d) is full of entodermic cells. The uni-axial type of the mammal gastrula is accentuated in this way. However, opinions still differ considerably as to the real nature of this "provisional gastrula" of the placental and its relation to the blastula into which it is converted.

As the gastrulation proceeds a large spherical blastula is formed from this peculiar solid amphigastrula of the placental, as we saw in the case of the marsupial. The accumulation of fluid in the solid gastrula (Figure 1.73 A) leads to the formation of an eccentric cavity, the group of the darker entodermic cells (hy) remaining directly attached at one spot with the round enveloping stratum of the lighter ectodermic cells (ep). This spot corresponds to the original primitive mouth (prostoma or blastoporus). From this important spot the inner germinal layer spreads all round on the inner surface of the outer layer, the cell-stratum of which forms the wall of the hollow sphere; the extension proceeds from the vegetal towards the animal pole.

(FIGURE 1.67. Longitudinal section through the oval gastrula of the opossum (Figure 1.69). (From Selenka.) p primitive mouth, e ectoderm, i entoderm, d yelk remains in the primitive gut-cavity (u).)

The cenogenetic gastrulation of the placental has been greatly modified by secondary adaptation in the various groups of this most advanced and youngest sub-class of the mammals. Thus, for instance, we find in many of the rodents (guinea-pigs, mice, etc.) APPARENTLY a temporary inversion of the two germinal layers. This is due to a folding of the blastodermic wall by what is called the "girder," a plug-shaped growth of Rauber's "roof-layer." It is a thin layer of flat epithelial cells, that is freed from the surface of the blastoderm in some of the rodents; it has no more significance in connection with the general course of placental gastrulation than the conspicuous departure from the usual globular shape in the blastula of some of the ungulates. In some pigs and ruminants it grows into a thread-like, long and thin tube.

(FIGURE 1.68. Stem-cell of the mammal ovum (from the rabbit). k stem-nucleus, n nuclear corpuscle, p protoplasm of the stem-cell, z modified zona pellucida, h outer albuminous membrane, s dead sperm-cells.

FIGURE 1.69. Incipient cleavage of the mammal ovum (from the rabbit). The stem-cell has divided into two unequal cells, one lighter (e) and one darker (i). z zona pellucida, h outer albuminous membrane, s dead sperm-cell.

FIGURE 1.70. The first four segmentation-cells of the mammal ovum (from the rabbit). e the two larger (and lighter) cells, i the two smaller (and darker) cells, z zona pellucida, h outer albuminous membrane.

FIGURE 1.71. Mammal ovum with eight segmentation-cells (from the rabbit). e four larger and lighter cells, i four smaller and darker cells, z zona pellucida, h outer albuminous membrane.)

Thus the gastrulation of the placentals, which diverges most from that of the amphioxus, the primitive form, is reduced to the original type, the invagination of a modified blastula. Its chief peculiarity is that the folded part of the blastoderm does not form a completely closed (only open at the primitive mouth) blind sac, as is usual; but this blind sac has a wide opening at the ventral curve (opposite to the dorsal mouth); and through this opening the primitive gut communicates from the first with the embryonic cavity of the blastula. The folded crest-shaped entoderm grows with a free circular border on the inner surface of the entoderm towards the vegetal pole; when it has reached this, and the inner surface of the blastula is completely grown over, the primitive gut is closed. This remarkable direct transition of the primitive gut-cavity into the segmentation-cavity is explained simply by the assumption that in most of the mammals the yelk-mass, which is still possessed by the oldest forms of the class (the monotremes) and their ancestors (the reptiles), is atrophied. This proves the essential unity of gastrulation in all the vertebrates, in spite of the striking differences in the various classes.

In order to complete our consideration of the important processes of segmentation and gastrulation, we will, in conclusion, cast a brief glance at the fourth chief type--superficial segmentation. In the vertebrates this form is not found at all. But it plays the chief part in the large stem of the articulates--the insects, spiders, myriapods, and crabs. The distinctive form of gastrula that comes of it is the "vesicular gastrula" (Perigastrula).

In the ova which undergo this superficial cleavage the formative yelk is sharply divided from the nutritive yelk, as in the preceding cases of the ova of birds, reptiles, fishes, etc.; the formative yelk alone undergoes cleavage. But while in the ova with discoid gastrulation the formative yelk is not in the centre, but at one pole of the uni-axial ovum, and the food-yelk gathered at the other pole, in the ova with superficial cleavage we find the formative yelk spread over the whole surface of the ovum; it encloses spherically the food-yelk, which is accumulated in the middle of the ova. As the segmentation only affects the former and not the latter, it is bound to be entirely "superficial"; the store of food in the middle is quite untouched by it. As a rule, it proceeds in regular geometrical progression. In the end the whole of the formative yelk divides into a number of small and homogeneous cells, which lie close together in a single stratum on the entire surface of the ovum, and form a superficial blastoderm. This blastoderm is a simple, completely closed vesicle, the internal cavity of which is entirely full of food-yelk. This real blastula only differs from that of the primitive ova in its chemical composition. In the latter the content is water or a watery jelly; in the former it is a thick mixture, rich in food-yelk, of albuminous and fatty substances. As this quantity of food-yelk fills the centre of the ovum before cleavage begins, there is no difference in this respect between the morula and the blastula. The