The Evolution of Man, vol 1 by Ernst Haeckel (paper ebook reader .txt) 📕

(FIGURES 1.83 AND 1.84. Chordula of the amphioxus. Figure 1.83 median longitudinal section (seen from the left). Figure 1.84 transverse section. (From Hatschek.) In Figure 1.83 the coelom-pouches are omitted, in order to show the chordula more clearly. Figure 1.84 is rather diagrammatic. h horny-plate, m medullary tube, n wall of same (n apostrophe, dorsal, n double apostrophe, ventral), ch chorda, np neuroporus, ne canalis neurentericus, d gut-cavity, r gut dorsal wall, b gut ventral wall, z yelk-cells in the latter, u primitive mouth, o mouth-pit, p promesoblasts (primitive or polar cells of the mesoderm), w parietal layer, v visceral layer of the mesoderm, c coelom, f rest of the segmentation-cavity.

FIGURES 1.85 AND 1.86. Chordula of the amphibia (the ringed adder).

(From Goette.) Figure 85 median longitudinal section (seen from the left), Figure 1.86 transverse section (slightly diagrammatic).

Lettering as in Figures 1.83 and 1.84.

FIGURES 1.87 AND 1.88. Diagrammatic vertical section of coelomula-embryos of vertebrates. (From Hertwig.) Figure 1.87, vertical section THROUGH the primitive mouth, Figure 1.88, vertical section BEFORE the primitive mouth. u primitive mouth, ud primitive gut. d yelk, dk yelk-nuclei, dh gut-cavity, lh body-cavity, mp medullary plate, ch chorda plate, ak and ik outer and inner germinal layers, pb parietal and vb visceral mesoblast.

FIGURES 1.89 AND 1.90. Transverse section of coelomula embryos of triton. (From Hertwig.) Figure 1.89, section THROUGH the primitive mouth. Figure 1.90, section in front of the primitive mouth, u primitive mouth. dh gut-cavity, dz yelk-cells, dp yelk-stopper, ak outer and ik inner germinal layer, pb parietal and vb visceral middle layer, m medullary plate, ch chorda.)

I give the name of chordula or chorda-larva to the embryonic stage of the vertebrate organism which is represented by the amphioxus larva at this period (Figures 1.83 and 1.84, in the third period of development according to Hatschek). (Strabo and Plinius give the name of cordula or cordyla to young fish larvae.) I ascribe the utmost phylogenetic significance to it, as it is found in all the chorda-animals (tunicates as well as vertebrates) in essentially the same form.

Although the accumulation of food-yelk greatly modifies the form of the chordula in the higher vertebrates, it remains the same in its main features throughout. In all cases the nerve-tube (m) lies on the dorsal side of the bilateral, worm-like body, the gut-tube (d) on the ventral side, the chorda (ch) between the two, on the long axis, and the coelom pouches (c) at each side. In every case these primitive organs develop in the same way from the germinal layers, and the same organs always arise from them in the mature chorda-animal. Hence we may conclude, according to the laws of the theory of descent, that all these chordonia or chordata (tunicates and vertebrates) descend from an ancient common ancestral form, which we may call Chordaea. We should regard this long-extinct Chordaea, if it were still in existence, as a special class of unarticulated worm (chordaria). It is especially noteworthy that neither the dorsal nerve-tube nor the ventral gut-tube, nor even the chorda that lies between them, shows any trace of articulation or segmentation; even the two coelom-sacs are not segmented at first (though in the amphioxus they quickly divide into a series of parts by transverse folding). These ontogenetic facts are of the greatest importance for the purpose of learning those ancestral forms of the vertebrates which we have to seek in the group of the unarticulated vermalia. The coelom-pouches were originally sexual glands in these ancient chordonia.

(FIGURE 1.91. A, B, C. Vertical section of the dorsal part of three triton-embryos. (From Hertwig.) In Figure A the medullary swellings (the parallel borders of the medullary plate) begin to rise; in Figure B they grow towards each other; in Figure C they join and form the medullary tube. mp medullary plate, mf medullary folds, n nerve-tube, ch chorda, lh body-cavity, mk1 and mk2 parietal and visceral mesoblasts, uv primitive-segment cavities, ak ectoderm, ik entoderm, dz yelk-cells, dh gut-cavity.)

From the evolutionary point of view the coelom-pouches are, in any case, older than the chorda; since they also develop in the same way as in the chordonia in a number of invertebrates which have no chorda (for instance, Sagitta, Figures 1.76 to 1.78). Moreover, in the amphioxus the first outline of the chorda appears later than that of the coelom-sacs. Hence we must, according to the biogenetic law, postulate a special intermediate form between the gastrula and the chordula, which we will call coelomula, an unarticulated, worm-like body with primitive gut, primitive mouth, and a double body-cavity, but no chorda. This embryonic form, the bilateral coelomula (Figure 1.81), may in turn be regarded as the ontogenetic reproduction (maintained by heredity) of an ancient ancestral form of the coelomaria, the Coelomaea (cf. Chapter 2.20).

In Sagitta and other worm-like animals the two coelom-pouches (presumably gonads or sex-glands) are separated by a complete median partition, the dorsal and ventral mesentery (Figure 1.78 dm and vm); but in the vertebrates only the upper part of this vertical partition is maintained, and forms the dorsal mesentery. This mesentery afterwards takes the form of a thin membrane, which fastens the visceral tube to the chorda (or the vertebral column). At the under side of the visceral tube the coelom-sacs blend together, their inner or median walls breaking down and disappearing. The body-cavity then forms a single simple hollow, in which the gut is quite free, or only attached to the dorsal wall by means of the mesentery.

The development of the body-cavity and the formation of the chordula in the higher vertebrates is, like that of the gastrula, chiefly modified by the pressure of the food-yelk on the embryonic structures, which forces its hinder part into a discoid expansion. These cenogenetic modifications seem to be so great that until twenty years ago these important processes were totally misunderstood. It was generally believed that the body-cavity in man and the higher vertebrates was due to the division of a simple middle layer, and that the latter arose by cleavage from one or both of the primary germinal layers. The truth was brought to light at last by the comparative embryological research of the Hertwigs. They showed in their Coelom Theory (1881) that all vertebrates are true enterocoela, and that in every case a pair of coelom-pouches are developed from the primitive gut by folding. The cenogenetic chordula-forms of the craniotes must therefore be derived from the palingenetic embryology of the amphioxus in the same way as I had previously proved for their gastrula-forms.

The chief difference between the coelomation of the acrania (amphioxus) and the other vertebrates (with skulls—craniotes) is that the two coelom-folds of the primitive gut in the former are from the first hollow vesicles, filled with fluid, but in the latter are empty pouches, the layers of which (inner and outer) close with each other.

In common parlance we still call a pouch or pocket by that name, whether it is full or empty. It is different in ontogeny; in some of our embryological literature ordinary logic does not count for very much. In many of the manuals and large treatises on this science it is proved that vesicles, pouches, or sacs deserve that name only when they are inflated and filled with a clear fluid. When they are not so filled (for instance, when the primitive gut of the gastrula is filled with yelk, or when the walls of the empty coelom-pouches are pressed together), these vesicles must not be cavities any longer, but “solid structures.”

The accumulation of food-yelk in the ventral wall of the primitive gut (Figures 1.85 and 1.86) is the simple cause that converts the sac-shaped coelom-pouches of the acrania into the leaf-shaped coelom-streaks of the craniotes. To convince ourselves of this we need only compare, with Hertwig, the palingenetic coelomula of the amphioxus (Figures 1.80 and 1.81) with the corresponding cenogenetic form of the amphibia (Figures 1.89 to 1.90), and construct the simple diagram that connects the two (Figures 1.87 and 1.88). If we imagine the ventral half of the primitive gut-wall in the amphioxus embryo (Figures 1.79 to 1.84) distended with food-yelk, the vesicular coelom-pouches (lh) must be pressed together by this, and forced to extend in the shape of a thin double plate between the gut-wall and body-wall (Figures 1.86 and 1.87). This expansion follows a downward and forward direction. They are not directly connected with these two walls. The real unbroken connection between the two middle layers and the primary germ-layers is found right at the back, in the region of the primitive mouth (Figure 1.87 u). At this important spot we have the source of embryonic development (blastocrene), or “zone of growth,” from which the coelomation (and also the gastrulation) originally proceeds.

(FIGURE 1.92. Transverse section of the chordula-embryo of a bird (from a hen’s egg at the close of the first day of incubation). (From Kolliker,) h horn-plate (ectoderm), m medullary plate, Rf dorsal folds of same, Pv medullary furrow, ch chorda, uwp median (inner) part of the middle layer (median wall of the coelom-pouches), sp lateral (outer) part of same, or lateral plates, uwh structure of the body-cavity, dd gut-gland-layer.)

Hertwig even succeeded in showing, in the coelomula-embryo of the water salamander (Triton), between the first structures of the two middle layers, the relic of the body-cavity, which is represented in the diagrammatic transitional form (Figures 1.87 and 1.88). In sections both through the primitive mouth itself (Figure 1.89) and in front of it (Figure 1.90) the two middle layers (pb and vb) diverge from each other, and disclose the two body-cavities as narrow clefts.

At the primitive-mouth itself (Figure 1.90 u) we can penetrate into them from without. It is only here at the border of the primitive mouth that we can show the direct transition of the two middle layers into the two limiting layers or primary germinal layers.

The structure of the chorda also shows the same features in these coelomula-embryos of the amphibia (Figure 1.91) as in the amphioxus (Figures 1.79 to 1.82). It arises from the entodermic cell-streak, which forms the middle dorsal-line of the primitive gut, and occupies the space between the flat coelom-pouches (Figure 1.91 A). While the nervous centre is formed here in the middle line of the back and separated from the ectoderm as “medullary tube,” there takes place at the same time, directly underneath, the severance of the chorda from the entoderm (Figure 1.91 A, B, C). Under the chorda is formed (out of the ventral entodermic half of the gastrula) the permanent gut or visceral cavity (enteron) (Figure 1.91 B, dh). This is done by the coalescence, under the chorda in the median line, of the two dorsal side-borders of the gut-gland-layer (ik), which were previously separated by the chorda-plate (Figure 1.91 A, ch); these now alone form the clothing of the visceral cavity (dh) (enteroderm, Figure 1.91

C). All these important modifications take place at first in the fore or head-part of the embryo, and spread backwards from there; here at the hinder end, the region of the primitive mouth, the important border of the mouth (or properistoma) remains for a long time the source of development or the zone of fresh construction, in the further building-up of the organism. One has only to compare carefully the illustrations given (Figures 1.85 to 1.91) to see that, as a fact, the cenogenetic coelomation of the amphibia can be deduced directly from the palingenetic form of the acrania (Figures 1.79 to 1.84).

(FIGURE 1.93. Transverse section of the vertebrate-embryo of a bird (from a hen’s egg on the second day of incubation). (From Kolliker.) h horn-plate, mr medullary tube, ch chorda, uw primitive segments, uwh primitive-segment cavity (median relic of the coelom), sp lateral coelom-cleft, hpl skin-fibre-layer, df gut-fibre-layer, ung primitive-kidney passage, ao primitive aorta, dd gut-gland-layer.) The same principle holds good for the amniotes, the reptiles,