Among the fishes the selachii, or primitive fishes, yield the most important information on these and many other phylogenetic questions (Figures 1.161 and 1.162). The careful studies of Ruckert, 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 myocoels, h) are still connected with the ventral body-cavity (lh; Figure 1.161). In Figure 1.162, 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 coelom-pouches and the rise of the primitive segments from their dorsal half (cf. Figure 1.91, A, B, C). A horizontal longitudinal section of the salamander-embryo (Figure 1.163) 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. (FIGURE 1.164. The third cervical vertebra (human). FIGURE 1.165. The sixth dorsal vertebra (human). FIGURE 1.166. The second lumbar vertebra (human).) The metamerism of the amniotes agrees in all essential points with that of the three lower classes of vertebrates we have considered; but it varies considerably in detail, in consequence of cenogenetic disturbances that are due in the first place (like the degeneration of the coelom-pouches) to the large development of the food-yelk. As the pressure of this seems to force the two middle layers together from the start, and as the solid structure of the mesoderm apparently belies the original hollow character of the sacs, the two sections of the mesoderm, which are at that time divided by the lateral fold--the dorsal segment-plates and ventral side-plates--have the appearance at first of solid layers of cells (Figures 1.94 to 1.97). And when the articulation of the somites begins in the sole-shaped embryonic shield, and a couple of protovertebrae are developed in succession, constantly increasing in number towards the rear, these cube-shaped somites (formerly called protovertebrae, or primitive vertebrae) have the appearance of solid dice, made up of mesodermic cells (Figure 1.93). Nevertheless, there is for a time a ventral cavity, or provertebral cavity, even in these solid "protovertebrae" (Figure 1.143 uwh). This vesicular condition of the provertebra is of the greatest phylogenetic interest; we must, according to the coelom theory, regard it as an hereditary reproduction of the hollow dorsal somites of the amphioxus (Figures 1.156 to 1.160) and the lower