The Evolution of Man, vol 1 - Ernst Haeckel (funny books to read .TXT) 📗

two sides of which (right and left) a couple of pouches are formed. As these coelom-pouches or coelom-sacs detach themselves from the primitive gut, a double body-cavity is formed (Figures 1.74 to 1.76).

(FIGURE 1.77. Coelomula of sagitta, in section. (From Hertwig.) D

dorsal side, V ventral side, ik inner germinal layer, mv visceral mesoblast, lh body-cavity, mp parietal mesoblast, ak outer germinal layer.)

The same kind of coelom-formation as in sagitta was afterwards found by Kowalevsky in brachiopods and other invertebrates, and in the lowest vertebrate—the amphioxus. Further instances were discovered by two English embryologists, to whom we owe very considerable advance in ontogeny—E. Ray-Lankester and F. Balfour. On the strength of these and other studies, as well as most extensive research of their own, the brothers Oscar and Richard Hertwig constructed in 1881 the Coelom Theory. In order to appreciate fully the great merit of this illuminating and helpful theory, one must remember what a chaos of contradictory views was then represented by the “problem of the mesoderm,” or the much-disputed “question of the origin of the middle germinal layer.” The coelom theory brought some light and order into this infinite confusion by establishing the following points: 1. The body-cavity originates in the great majority of animals (especially in all the vertebrates) in the same way as in sagitta: a couple of pouches or sacs are formed by folding inwards at the primitive mouth, between the two primary germinal layers; as these pouches detach from the primitive gut, a pair of coelom-sacs (right and left) are formed; the coalescence of these produces a simple body-cavity. 2. When these coelom-embryos develop, not as a pair of hollow pouches, but as solid layers of cells (in the shape of a pair of mesodermal streaks)—as happens in the higher vertebrates—we have a secondary (cenogenetic) modification of the primary (palingenetic) structure; the two walls of the pouches, inner and outer, have been pressed together by the expansion of the large food-yelk. 3. Hence the mesoderm consists from the first of TWO genetically distinct layers, which do not originate by the cleavage of a primary simple middle layer (as Remak supposed).

4. These two middle layers have, in all vertebrates, and the great majority of the invertebrates, the same radical significance for the construction of the animal body; the inner middle layer, or the visceral mesoderm, (gut-fibre layer), attaches itself to the original entoderm, and forms the fibrous, muscular, and connective part of the visceral wall; the outer middle layer, or the parietal mesoderm (skin-fibre-layer), attaches itself to the original ectoderm and forms the fibrous, muscular, and connective part of the body-wall. 5. It is only at the point of origination, the primitive mouth and its vicinity, that the four secondary germinal layers are directly connected; from this point the two middle layers advance forward separately between the two primary germinal layers, to which they severally attach themselves. 6. The further separation or differentiation of the four secondary germinal layers and their division into the various tissues and organs take place especially in the later fore-part or head of the embryo, and extend backwards from there towards the primitive mouth.

(FIGURE 1.78. Section of a young sagitta. (From Hertwig.) dh visceral cavity, ik and ak inner and outer limiting layers, mv and mp inner and outer middle layers, lk body-cavity, dm and vm dorsal and visceral mesentery.)

All animals in which the body-cavity demonstrably arises in this way from the primitive gut (vertebrates, tunicates, echinoderms, articulates, and a part of the vermalia) were comprised by the Hertwigs under the title of enterocoela, and were contrasted with the other groups of the pseudocoela (with false body-cavity) and the coelenterata (with no body-cavity). However, this radical distinction and the views as to classification which it occasioned have been shown to be untenable. Further, the absolute differences in tissue-formation which the Hertwigs set up between the enterocoela and pseudocoela cannot be sustained in this connection. For these and other reasons their coelom-theory has been much criticised and partly abandoned.

Nevertheless, it has rendered a great and lasting service in the solution of the difficult problem of the mesoderm, and a material part of it will certainly be retained. I consider it an especial merit of the theory that it has established the identity of the development of the two middle layers in all the vertebrates, and has traced them as cenogenetic modifications back to the original palingenetic form of development that we still find in the amphioxus. Carl Rabl comes to the same conclusion in his able Theory of the Mesoderm, and so do Ray-Lankester, Rauber, Kupffer, Ruckert, Selenka, Hatschek, and others. There is a general agreement in these and many other recent writers that all the different forms of coelom-construction, like those of gastrulation, follow one and the same strict hereditary law in the vast vertebrate stem; in spite of their apparent differences, they are all only cenogenetic modifications of one palingenetic type, and this original type has been preserved for us down to the present day by the invaluable amphioxus.

(FIGURES 1.79 AND 1.80. Transverse section of amphioxus-larvae. (From Hatschek.) Figure 1.79 at the commencement of coelom formation (still without segments), Figure 1.80 at the stage with four primitive segments. ak, ik, mk outer, inner, and middle germinal layer, hp horn plate, mp medullary plate, ch chorda, asterisk and asterisk, disposition of the coelom-pouches, lh body-cavity.) But before we go into the regular coelomation of the amphioxus, we will glance at that of the arrow-worm (Sagitta), a remarkable deep-sea worm that is interesting in many ways for comparative anatomy and ontogeny. On the one hand, the transparency of the body and the embryo, and, on the other hand, the typical simplicity of its embryonic development, make the sagitta a most instructive object in connection with various problems. The class of the chaetogatha, which is only represented by the cognate genera of Sagitta and Spadella, is in another respect also a most remarkable branch of the extensive vermalia stem. It was therefore very gratifying that Oscar Hertwig (1880) fully explained the anatomy, classification, and evolution of the chaetognatha in his careful monograph.

The spherical blastula that arises from the impregnated ovum of the sagitta is converted by a folding at one pole into a typical archigastrula, entirely similar to that of the Monoxenia which I described (Chapter 1.8, Figure 1.29). This oval, uni-axial cup-larva (circular in section) becomes bilateral (or tri-axial) by the growth of a couple of coelom-pouches from the primitive gut (Figures 1.76 and 1.77). To the right and left a sac-shaped fold appears towards the top pole (where the permanent mouth, m, afterwards arises). The two sacs are at first separated by a couple of folds of the entoderm (Figure 1.76 pv), and are still connected with the primitive gut by wide apertures; they also communicate for a short time with the dorsal side (Figure 1.77 d). Soon, however, the coelom-pouches completely separate from each other and from the primitive gut; at the same time they enlarge so much that they close round the primitive gut (Figure 1.78).

But in the middle line of the dorsal and ventral sides the pouches remain separated, their approaching walls joining here to form a thin vertical partition, the mesentery (dm and vm). Thus Sagitta has throughout life a double body-cavity (Figure 1.78 lk), and the gut is fastened to the body-wall both above and below by a mesentery—below by the ventral mesentery (vm), and above by the dorsal mesentery (dm).

The inner layer of the two coelom-pouches (mv) attaches itself to the entoderm (ik), and forms with it the visceral wall. The outer layer (mp) attaches itself to the ectoderm (ak), and forms with it the outer body-wall. Thus we have in Sagitta a perfectly clear and simple illustration of the original coelomation of the enterocoela. This palingenetic fact is the more important, as the greater part of the two body-cavities in Sagitta changes afterwards into sexual glands—the fore or female part into a pair of ovaries, and the hind or male part into a pair of testicles.

Coelomation takes place with equal clearness and transparency in the case of the amphioxus, the lowest vertebrate, and its nearest relatives, the invertebrate tunicates, the sea-squirts. However, in these two stems, which we class together as Chordonia, this important process is more complex, as two other processes are associated with it—the development of the chorda from the entoderm and the separation of the medullary plate or nervous centre from the ectoderm. Here again the skulless amphioxus has preserved to our own time by tenacious heredity the chief phenomena in their original form, while it has been more or less modified by embryonic adaptation in all the other vertebrates (with skulls). Hence we must once more thoroughly understand the palingenetic embryonic features of the lancelet before we go on to consider the cenogenetic forms of the craniota.

(FIGURES 1.81 AND 1.82. Transverse section of amphioxus embryo. Figure 1.81 at the stage with five somites, Figure 1.82 at the stage with eleven somites. (From Hatschek.) ak outer germinal layer, mp medullary plate, n nerve-tube, ik inner germinal layer, dh visceral cavity, lh body-cavity, mk middle germinal layer (mk1 parietal, mk2 visceral), us primitive segment, ch chorda.)

The coelomation of the amphioxus, which was first observed by Kowalevsky in 1867, has been very carefully studied since by Hatschek (1881). According to him, there are first formed on the bilateral gastrula we have already considered (Figures 1.36 and 1.37) three parallel longitudinal folds—one single ectodermal fold in the central line of the dorsal surface, and a pair of entodermic folds at the two sides of the former. The broad ectodermal fold that first appears in the middle line of the flattened dorsal surface, and forms a shallow longitudinal groove, is the beginning of the central nervous system, the medullary tube. Thus the primary outer germinal layer divides into two parts, the middle medullary plate (Figure 1.81 mp) and the horny-plate (ak), the beginning of the outer skin or epidermis. As the parallel borders of the concave medullary plate fold towards each other and grow underneath the horny-plate, a cylindrical tube is formed, the medullary tube (Figure 1.82 n); this quickly detaches itself altogether from the horny-plate. At each side of the medullary tube, between it and the alimentary tube (Figures 1.79 to 1.82 dh), the two parallel longitudinal folds grow out of the dorsal wall of the alimentary tube, and these form the two coelom-pouches (Figures 1.80

and 1.81 lh). This part of the entoderm, which thus represents the first structure of the middle germinal layer, is shown darker than the rest of the inner germinal layer in Figures 1.79 to 1.82. The edges of the folds meet, and thus form closed tubes (Figure 1.81 in section).

During this interesting process the outline of a third very important organ, the chorda or axial rod, is being formed between the two coelom-pouches. This first foundation of the skeleton, a solid cylindrical cartilaginous rod, is formed in the middle line of the dorsal primitive gut-wall, from the entodermal cell-streak that remains here between the two coelom-pouches (Figures 1.79 to 1.82 ch).

The chorda appears at first in the shape of a flat longitudinal fold or a shallow groove (Figures 1.80 and 1.81); it does not become a solid cylindrical cord until after separation from the primitive gut (Figure 1.82). Hence we might say that the dorsal wall of the primitive gut forms three parallel longitudinal folds at this important period—one single fold and a pair of folds. The single middle fold becomes the chorda, and lies immediately below the groove of the ectoderm, which becomes the medullary tube; the pair of folds to the right and left lie at the sides between the former and the latter, and form the coelom-pouches. The part of the primitive gut that remains after the cutting off of these three dorsal primitive organs is the permanent gut;