400 



THE EVOLUTION OF MAN. 



remainder of the body-cavity (coeloma). Of tliese latter — 

 the true ccjelom -cells — those which invest the intestinal 

 tube and the mesentery (" endocoelar ") 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 tirjies : h, horn-plate ; w, medullary tube ; n, canal of the medullary 

 tube ; u, primitive kidneys ; x, notochord ; e, hind limbs ; b, allantois canal 

 in ventral wall; t, aorta; v, cardinal veins'; a, intestine; d, intestinal- 

 glandular layer; /, intestinal-fibrous layer; g, germ-epithelium • r, dorsal 

 muscles ; c, body-cavity, or Coelom. (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 (" exocoelar ") are, on the contrary, the product 

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

 but the sexual cells which make their appearance at the 

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



