Fig.157. Embryo of the amphioxus, twenty hours old, with five somites. Fig. 157—Embryo of the amphioxus, twenty hours old, with five somites. (Right view; for left view see Fig. 124.) (From _Hatschek._) _ V_ fore end, _H_ hind end. _ak, mk, ik_ outer, middle, and inner germinal layers; _dh_ alimentary canal, _n_ neural tube, _cn_ canalis neurentericus, _ush_ cœlom-pouches (or primitive-segment cavities), _us1_ first (and foremost) primitive segment.
The number of the metamera, and of the embryonic somites or primitive segments from which they develop, varies considerably in the vertebrates, according as the hind part of the body is short or is lengthened by a tail. In the developed man the trunk (including the rudimentary tail) consists of thirty-three metamera, the solid centre of which is formed by that number of vertebræ in the vertebral column (seven cervical, twelve dorsal, five lumbar, five sacral, and four caudal). To these we must add at least nine head-vertebræ, which originally (in all the craniota) constitute the skull. Thus the total number of the primitive segments of the human body is raised to at least forty-two; it would reach forty-five to forty-eight if (according to recent investigations) the number of the original segments of the skull is put at twelve to fifteen. In the tailless or anthropoid apes the number of metamera is much the same as in man, only differing by one or two; but it is much larger in the long-tailed apes and most of the other mammals. In long serpents and fishes it reaches several hundred (sometimes 400).
Figs. 158-160. Embryo of the amphioxus, twenty four hours old, with eight somites. Figs. 158–160—Embryo of the amphioxus, twenty four hours old, with eight somites. (From _Hatschek._) Figs. 158 and 159 lateral view (from left). Fig. 160 seen from back. In Fig. 158 only the outlines of the eight primitive segments are indicated, in Fig. 159 their cavities and muscular walls. _V_ fore end, _H_ hind end, _d_ gut, _du_ under and _ dd_ upper wall of the gut, _ne_ canalis neurentericus, _ nv_ ventral, _nd_ dorsal wall of the neural tube, _np_ neuroporus, _dv_ fore pouch of the gut, _ch_ chorda, _ mf_ mesodermic fold, _pm_ polar cells of the mesoderm (_ms_), _e_ ectoderm.