The articulation proceeds in substantially the same way in the other vertebrates, the craniota, starting from the cœlom-pouches. But whereas in the former case there is first a transverse division of the cœlom-sacs (by vertical folds) and then the dorso-ventral division, the procedure is reversed in the craniota; in their case each of the long cœlom-pouches first divides into a dorsal (primitive segment plates) and a ventral (lateral plates) section by a lateral longitudinal fold. Only the former are then broken up into primitive segments by the subsequent vertical folds; while the latter (segmented for a time in the amphioxus) remain undivided, and, by the divergence of their parietal and visceral plates, form a body-cavity that is unified from the first. In this case, again, it is clear that we must regard the features of the younger craniota as cenogenetically modified processes that can be traced palingenetically to the older acrania.
We have an interesting intermediate stage between the acrania and the fishes in these and many other respects in the cyclostoma (the hag and the lamprey, cf. Chapter XXI).
Fig. 163—Frontal (or horizontal-longitudinal) section of a triton-embryo with three pairs of primitive segments. ch chorda, us primitive segments, ush their cavity, ak horn plate.
Among the fishes the selachii, or primitive fishes, yield the most important information on these and many other phylogenetic questions (Figs. 161 and 162). The careful studies of Rückert, Van Wijhe, H. E. Ziegler, and others, have given us most valuable results. The products of the middle germinal layer are partly clear in these cases at the period when the dorsal primitive segment cavities (or myocœls, h) are still connected with the ventral body-cavity (lh; Fig. 161). In Fig. 162, a somewhat older embryo, these cavities are separated. The outer or lateral wall of the dorsal segment yields the cutis-plate (cp), the foundation of the connective corium. From its inner or median wall are developed the muscle-plate (mp, the rudiment of the trunk-muscles) and the skeletal plate, the formative matter of the vertebral column (sk).
In the amphibia, also, especially the water-salamander (Triton), we can observe very clearly the articulation of the cœlom-pouches and the rise of the primitive segments from their dorsal half (cf. Fig. 91, A, B, C). A horizontal longitudinal section of the salamander-embryo (Fig. 163) shows very clearly the series of pairs of these vesicular dorsal segments, which have been cut off on each side from the ventral side-plates, and lie to the right and left of the chorda.
Fig. 164—The third cervical vertebra (human).
Fig. 165—The sixth dorsal vertebra (human).
Fig. 166—The second lumbar vertebra (human).