224 THE EVOLUTION OF MAN. 



centre of the yellow yelk -mass, and there forms a small 

 central ball of white yelk (Fig. 44, d). The whole mass of 

 this white yelk is, however, not sharply divided from the 

 yellow yelk, which in hardened eggs shows a slight trace 

 of concentric stratification (Fig. 44, c). Just as in this 

 globular egg in the ovary, so also in the hen's egg after 

 it has been laid ; when the egg-shell is opened and the 

 yelk taken out, a small, circular, white disc is seen ou 

 the upper surface of the latter. This disc represents the 

 cicatricula, or tread. This small white germ-disc is, how- 

 ever, far advanced in development, and is, in fact, the 

 Gastrula of the hen. The body of the latter proceeds 

 entirely from this Gastrula. The whole mass of white and 

 yellow yelk is entirely without share in the formation of the 

 Chick, for it is only used up as nutritive matter and con- 

 sumed as food by the embryo in the course of its evolution. 

 The transparent, tough, and voluminous mass of albumen, 

 surrounding the yellow yelk of the Bird's egg, and the hard 

 chalky shell of the egg, are formed round the egg, in the 

 oviduct, after it is already fertilized. 



After the fertilization of the egg within the body of the 

 parent Bird is complete, the germ-vesicle (vesicula ger- 

 minativa) probably, as in other cases, first disappears ; and 

 the reconstruction of a kernel results in a parent-cell 

 (cytula). This lentil-shaped parent-cell now undergoes a 

 discoidal cleavage (segvientatio discoidaUs, Fig. 45) entirely 

 similar to that of the egg of the Fish (Plate III. Fig. 18-24). 

 Two similar cleavage-cells (A) first arise from the parent- 

 ceU. These part into 4 (B), into 8, 16 (C), 32, 64 ceUs, and 

 so on. As before, the division of the kernel always precedes 

 the division of the cells. The planes of division between 



II 



