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. 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. (FIGURES 1.168 AND 1.169. Head of a chick embryo, of the third day. Figure 1.168 from the front, Figure 1.169 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 Kolliker.))