THE EVOLUTION OF MAN. 



skin-fibrous layer (Figs. 141, 142). The latter is indeed here 

 very thin and delicate, but yet can be distinctly shown to 

 be a direct continuation of the leather-skin (corium), and 



FIG. 141. Transverse section through an embryonic Chick in the navel 

 region (at the fifth day of incubation). The amnion-folds (am) almost 

 meet over the back of the embryo. The intestine (d), still open, passes 

 below into the yelk-sac : df, intestinal-fibrous layer ; sh, notochord ; sa, 

 aorta ; vc, principal veins ; Ih, ventral cavity, not yet closed ; v, anterior, 

 g, posterior nerve-roots of the spinal marrow j mu, muscle-plate ; hp, 

 leather-plate ; h, horn-plate. (After Kemak.) 



is, therefore, the outermost layer arising from the fission of 

 the middle germ-layer (mesoderma). Thus the outer 

 peripheric portion of the skin-fibrous layer clothes only the 

 inner lamella of the amnion-fold (the head-sheath, tail- 

 sheath, etc.), and extends only to the edge of the fold itself. 

 The outer lamella is formed entirely by the horn-plate, and 



