22O THE EVOLUTION OF MAN. 



surface of this disc lies immediately on the upper, convex 

 surface of the nutritive yelk (TI). 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 egg, 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 (proper istoma), the 

 entoderm, and the exoderm pass into each other without 

 clear limits (Fig. 43, w). 



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 swallow 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- 



