These concordant results of the best recent investigations (which have been confirmed by the observations of a number of scientists I have not enumerated) prove the unity of the vertebrate-stem in point of cœlomation, no less than of gastrulation. In both respects the invaluable amphioxus—the sole survivor of the acrania—is found to be the original model that has preserved for us in palingenetic form by a tenacious heredity these most important embryonic processes. From this primary model of construction we can cenogenetically deduce all the embryonic forms of the other vertebrates, the craniota, by secondary modifications. My thesis of the universal formation of the gastrula by folding of the blastula has now been clearly proved for all the vertebrates; so also has been Hertwig’s thesis of the origin of the middle germinal layers by the folding of a couple of cœlom-pouches which appear at the border of the primitive mouth. Just as the gastræa-theory explains the origin and identity of the two primary layers, so the cœlom-theory explains those of the four secondary layers. The point of origin is always the properistoma, the border of the original primitive mouth of the gastrula, at which the two primary layers pass directly into each other. Fig.96. Transverse section of the primitive groove (or primitive mouth) of a rabbit. Fig. 96—Transverse section of the primitive groove (or primitive mouth) of a rabbit. (From _Van Beneden._) _ pr_ primitive mouth, _ul_ lips of same (primitive lips), _ak_ and _ik_ outer and inner germinal layers, _mk_ middle germinal layer, _mp_ parietal layer, _mv_ visceral layer of the mesoderm. Fig.97. Transverse section of the primitive mouth (or groove) of a human embryo (at the coelomula stage). Fig. 97—Transverse section of the primitive mouth (or groove) of a human embryo (at the cœlomula stage). (From _Count Spee._) _pr_ primitive mouth, _ul_ lips of same (primitive folds), _ak_ and _ik_ outer and inner germinal layers, _mk_ middle layer, _mp_ parietal layer, _mv_ visceral layer of the mesoblasts. Moreover, the cœlomula is important as the immediate source of the chordula, the embryonic reproduction of the ancient, typical, unarticulated, worm-like form, which has an axial chorda between the dorsal nerve-tube and the ventral gut-tube. This instructive chordula (Figs. 83–86) provides a valuable support of our phylogeny; it indicates the important moment in our stem-history at which the stem of the chordonia (tunicates and vertebrates) parted for ever from the divergent stems of the other metazoa (articulates, echinoderms, and molluscs).