62 



THE EVOLUTION OF MAN. 



which are arranged in a simple strata-like way, and extend 

 over the outer and inner surface of the body as a protective 

 and secreting cover. This is proved by the simple fact that 

 the formation of the tissues of the animal body begins 

 with the formation of the gastrula, and that the latter 

 itself consists solely of two simple epithelial strata, of the 

 skin-layer (Fig. 274, e), and of the intestinal-layer (i). 

 Histologically, the two primary germ-layers are simple 

 epithelia. When these, afterwards, separate into the four 

 secondary germ-layers, the skin-sensory layer becomes the 

 outermost of the external coverings (dermal-epithelium) ; 

 the intestinal-glandular layer becomes the innermost of the 

 internal coverings (gastral-epithelium). The tissue of the 

 outer skin and of all its appendages, such as nails (Fig. 289), 



Fig. 289. — Tissue of the nails (flattened epithelium) : a-e, cells of the 

 upper strata ; /, g, cells of the lower strata. 



Fig. 290. — Tissue of the covering of the small intestine (columnar 

 epitheliun-t) : a, side view of three cells (with thicker, porous borders) ; h, 

 surface view of four cells. (After Frey.) 



hairs, skin-glands, etc., arise from the skin-sensory layer. 

 (Cf Table XXIX., p. 232.) The inner covering of the intes- 

 tinal tube and of its intestinal glands originates, on the 

 other hand, from the intestinal-glandular layer (Fig. 290). 



