400 



THE EVOLUTION OF MAN. 



remainder of the body-cavity (cceloma). Of these latter 

 the true ccelom-cells those which invest the intestinal 

 tube and the mesentery (" cndoccelar ") originate from the 



FIG. 316. Transverse section through the pelvic region and the hind 

 limbs of an embryo Chick in the fourth day of incubation, enlarged about 

 40 times : 7i, horn-plate ; w, medullary tube ; n, canal of the medullary 

 tube ; u, primitive kidneys ; a-, notochord ; c, hind limbs ; b, allantois canal 

 in ventral wall; *, aorta; v, cardinal veins; o, intestine; d, intestinal- 

 glandular layer; /, intestinal-fibrous layer; (?, germ-epithelinm ; r, dorsal 

 muscles ; c, body-cavity, or Coslom. (After Waldeyer.) 



intestinal-fibrous layer (in Fig. 5, Plate IV., coloured red) ; 

 those which line the inner surface of the external wall of 

 the abdomen (" exoccelar ") are, on the contrary, the product 

 of the skin-fibrous layer (coloured blue in Fig. 5, Plate IV.) 

 but the sexual cells which make their appearance at the 

 boundary line between the two forms of coelom-cells, and 



