STRUCTURE OF THE GASTRULA. 



195 



the primitive intestine, and alone constitute the wall of 

 the latter, are of very great significance. For these two 

 which alone constitute the whole body, are, in fact, the 

 two primary germ-layers, or primitive germ -layers (blas- 

 topliylla). Their fundamental significance has already been 

 pointed out in the historical introduction (Chapter III.). 

 The outer cell-layer is the skin-layer, or exoderm (Fig. 29, e) ; 

 the inner cell-layer is the intestinal layer, or entoderm 

 (Fig. 29, e). The whole body of all true animals proceeds 

 solely from these two primary germ-layers. The skin- 

 layer furnishes the outer body-wall; the intestinal layer 

 forms the inner wall of the intestine, and directly surrounds 

 the intestinal cavity. At a later period a cavity forms 



Fig. 29. — The Gastrula of a Chalk Sponge (Olynthus) : 4, external view. 

 B, in longitudinal section through the axis ; g, primitive intestine (primitive 

 intestinal cavity) ; o, primitive mouth (primitive mouth-opening) ; i, the 

 inner cell-layer of the body-wall (the inner germ-layer, entoderm or intes- 

 tinal layer) ; e, the outer cell-layer' (the outer germ-layer, exoderm or skin- 

 layer) . 



