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; its entoderm is the gut-gland-layer or enteric layer. (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.)