During this interesting process the outline of a third very important organ, the chorda or axial rod, is being formed between the two cœlom-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 cœlom-pouches (Figs. 79–82 _ch_). The chorda appears at first in the shape of a flat longitudinal fold or a shallow groove (Figs. 80, 81); it does not become a solid cylindrical cord until after separation from the primitive gut (Fig. 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 cœlom-pouches. The part of the primitive gut that remains after the cutting off of these three dorsal primitive organs is the permanent gut; its entoderm is the gut-gland-layer or enteric layer.
Figs. 83 and 84. Chordula of the amphioxus. 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). 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.