352 



THE EVOLUTION OF MAN. 



These cells are all the products of the skin-fibrous layer. 

 At a very early period, a small cavity appears in each of 

 these solid primitive vertebrae, which cavity, however, soon 

 again disappears. This " primitive vertebral cavity " (Figs. 

 95, 96, uivh, pp. 317, 318) is worthy of note only in so far as it 



FIG. 112. Transverse section through the embryo of a Chick oa the 

 fourth day of incubation (about 100 times the natural size). The primitive 

 vertebra? have separated into the outer muscle-plate (mp) and the inner 

 skeleton -plate. The latter below, as the vertebral bodies (v;ti), begins to 

 surround the notochord (c7i) ; above, as the vertebral arches (wl), begins 

 to surround the medullary tube (m), the cavity of which (mli), is already 

 very narrow. At wq the primitive vertebra passes into the skin-muscle 

 plate of the ventral wall (lip) ; Jipr, leather-plate of the dorsal wall ; ft, 

 horn-plate ; a, amnion ; ung t primitive kidney duct ; un, primitive urinary 

 canal ; ao, primitive artery ; vc, cardinal vein ; df, intestinal-fibrous layer ; 

 dd, intestinal-glandular layer ; dr, intestinal groove. 



