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



JK X 



Fig. 112. — Transverse section through the embryo of a Chick on the 

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

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

 skeleton -plate. The latter below, as the vertebral bodies {v)h), begins to 

 surround the notochord (ch) ; above, as the vertebral arches (tvh), begins 

 to suri'ound the medullary tube (m), the cavity of which (mh), is already 

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

 plate of the ventral wall (hp) ; hpr, leather-plate of the dorsal wall ; h, 

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

 canal; ao, primitive artery; vc, cardinal vein ; f?/, intestinal-fibrous layer; 

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



