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 cœlom-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 (Figs. 94–97). And when the articulation of the somites begins in the sole-shaped embryonic shield, and a couple of protovertebræ are developed in succession, constantly increasing in number towards the rear, these cube-shaped somites (formerly called protovertebræ, or primitive vertebræ) have the appearance of solid dice, made up of mesodermic cells (Fig. 93). Nevertheless, there is for a time a ventral cavity, or provertebral cavity, even in these solid “protovertebræ” (Fig. 143 uwh). This vesicular condition of the provertebra is of the greatest phylogenetic interest; we must, according to the cœlom theory, regard it as an hereditary reproduction of the hollow dorsal somites of the amphioxus (Figs. 156–160) and the lower vertebrates (Fig. 161–163). 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.
Fig. 167—Head of a shark embryo (Pristiurus), one-third of an inch long, magnified. (From Parker.) Seen from the ventral side.
The innermost median part of the primitive segment plates, which lies immediately on the chorda (Fig. 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 provertebræ 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 (Fig. 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 (Figs. 164–166). The upper or dorsal half of this bony ring, the vertebral arch (Fig. 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.
The whole of the secondary vertebra, which is thus formed from the union of the skeletal plates of two provertebral pieces and encloses a part of the chorda in its body, consists at first of a rather soft mass of cells; this afterwards passes into a firmer, cartilaginous stage, and finally into a third, permanent, bony stage. These three stages can generally be distinguished in the greater part of the skeleton of the higher vertebrates; at first most parts of the skeleton are soft, tender, and membranous; they then become cartilaginous in the course of their development, and finally bony.
Fig. 168 and 169—Head of a chick embryo, of the third day. Fig. 168 from the front, Fig. 169 from the right. n rudimentary nose (olfactory pit), l rudimentary eye (optic pit, lens-cavity), g rudimentary ear (auditory pit), v fore-brain, gl eye-cleft. Of the three pairs of gill-arches the first has passed into a process of the upper jaw (o) and of the lower jaw (u). (From Kölliker.)
At the head part of the embryo in the amniotes there is not generally a cleavage of the middle germinal layer into provertebral and lateral plates, but the dorsal and ventral somites are blended from the first, and form what are called the “head-plates” (Fig. 148 k). From these are formed the skull, the bony case of the brain, and the muscles and corium of the body. The skull develops in the same way as the membranous vertebral column. The right and left halves of the head curve over the cerebral vesicle, enclose the foremost part of the chorda below, and thus finally form a simple, soft, membranous capsule about the brain. This is afterwards converted into a cartilaginous primitive skull, such as we find permanently in many of the fishes. Much later this cartilaginous skull becomes the permanent bony skull with its various parts. The bony skull in man and all the other amniotes is more highly differentiated and modified than that of the lower vertebrates, the amphibia and fishes. But as the one has arisen phylogenetically from the other, we must assume that in the former no less than the latter the skull was originally formed from the sclerotomes of a number of (at least nine) head-somites.