220 THE EVOLUTION OF MAN. 



surface of this disc lies immediately on tlie upper, convex 

 surface of the nutritive yelk {n). On the other hand, the 

 outer surface of the disc is convex as in a Shark. If we 

 make a perpendicular section through a meridian-plane of 

 the globe-shaped ^gg, we shall find that it is composed of 

 several layers of cells (in this particular case there are four) 

 (Plate III. Fig. 24). Immediately above the nutritive yelk 

 lies a single layer of larger cells (Fig. 24, i), which are 

 characterized by a softer, less transparent, and more coarsely 

 granulated protoplasm, and which take up a dark red colour 

 from carmine. These form the intestinal layer, or entoderm, 

 which arises by the ingrowth of the edges of the disc 

 (infolded germ -layer). The three outer layers, lying on top 

 of this lower layer, form the skin-layer, or exoderm (Fig. 24, e). 

 They consist of smaller cells which take only a slight colour 

 from carmine ; their protoplasm is firmer, more transparent, 

 and more finely granulated. At the thickened edges of the 

 gastrula, the primitive mouth-edge {'properistoina), the 

 entoderm, and the exoderm pass into each other without 

 clear limits (Fig ^o, iv). 



It is evident that the most important peculiarities which 

 distinguish the Disc-gastrula from the two typical Gastrula- 

 forms which we before examined, are due to the large nutri- 

 tive yelk. This takes no part in the cleavage, and from the 

 first occupies the whole primitive intestinal cavity, while at 

 the same time it extends far beyond the mouth-opening of 

 the latter. If we imagine the original Bell-gastrula (Archi- 

 gastrula, Fig. 23-29) attempting to swalloAV a globe of 

 nutritive matter far larger than itself, in the attempt the 

 Gastrula will be spread out in the form of a disc on the 

 nutritive matter, much in the same way as in the Disc- 



