Figs. 83 and 84—Chordula of the amphioxus. Fig. 83 median longitudinal section (seen from the left). Fig. 84 transverse section. (From Hatschek.) In Fig. 83 the cœlom-pouches are omitted, in order to show the chordula more clearly. Fig. 84 is rather diagrammatic. h horny-plate, m medullary tube, n wall of same (n′ dorsal, n″ 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 cœlom, f rest of the segmentation-cavity.
Figs. 85 and 86—Chordula of the amphibia (the ringed adder). (From Goette.) Fig. 85 median longitudinal section (seen from the left), Fig. 86 transverse section (slightly diagrammatic). Lettering as in Figs. 83 and 84.
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 (Figs. 83, 84, in the third period of development according to Hatschek). (Strabo and Plinius give the name of cordula or cordyla to young fish larvæ.) 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 cœlom 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 Chordæa. We should regard this long-extinct Chordæa, 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 cœlom-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 cœlom-pouches were originally sexual glands in these ancient chordonia.
Figs. 87 and 88—Diagrammatic vertical section of cœlomula-embryos of vertebrates. (From Hertwig.) Fig. 87, vertical section through the primitive mouth, Fig. 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.