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 2.21).
(FIGURE 1.163. 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 (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 vertebrates (Figures 1.161 to 1.163). This rudimentary "provertebral cavity" has no physiological significance whatever in the amniote-embryo; it soon disappears, being filled up with cells of the muscular plate.
(FIGURE 1.167. Head of a shark embryo (Pristiurus), one-third of an inch long, magnified twenty times. (From Parker.) Seen from the ventral side.)
The innermost median part of the primitive segment plates, which lies immediately on the chorda (Figure 1.145 ch) and the medullary tube (m), forms the vertebral column in all the higher vertebrates (it is wanting in the lowest); hence it may be called the skeleton plate. In each of the provertebrae it is called the "sclerotome" (in opposition to the outlying muscular plate, the "myotome"). From the phylogenetic point of view the myotomes are much older than the sclerotomes. The lower or ventral part of each sclerotome (the inner and lower edge of the cube-shaped provertebra) divides into two plates, which grow round the chorda, and thus form the foundation of the body of the vertebra (wh). The upper plate presses between the chorda and the medullary tube, the lower between the chorda and the alimentary canal (Figure 1.137 C). As the plates of two opposite provertebral pieces unite from the right and left, a circular sheath is formed round this part of the chorda. From this develops the BODY of a vertebra—that is to say, the massive lower or ventral half of the bony ring, which is called the "vertebra" proper and surrounds the medullary tube (Figures 1.164 to 1.166). The upper or dorsal half of this bony ring, the vertebral arch (Figure 1.145 wb), arises in just the same way from the upper part of the skeletal plate, and therefore from the inner and upper edge of the cube-shaped primitive vertebra. As the upper edges of two opposing somites grow together over the medullary tube from right and left, the vertebra-arch becomes closed.