226 THE EVOLUTION OF MAN, 



an eight-rayed star. A circular furrow now forms round 

 the centre, so that the 8 three-cornered cleavage-cells 

 become 16, of which 8 lie in the middle, surrounded by 

 8 others {(J). After this, new furrows, some circular and 

 others radiating from the central point, succeed each other 

 more or less irregularly (D, E). Finally this cleavage- 

 process, like the others, results in the formation of small 

 cells of like character.'^^ In this case also, the cleavage- 

 cells form a circular lentil-shaped disc, which represents the 

 mulberry-germ, and lies embedded in a slight deepening in 

 the white yelk (Fig. 46, in perpendicular section). The 

 Morula in the case of the Hen's &gg is, however, thinner and 

 flatter than that of the egg of the Osseous Fish (Plate HI. 

 Fig. 21). 



In the Hen's egg, just as in that of the Osseous Fish, a 

 kenogenetic germ-vesicle, or Blastula, now arises (Fig. 47). 

 The cleavage-cells of the Morula increase in number and 

 move away from the nutritive-yelk, so that a disc, in 

 the form of a watch-glass, with thickened edges {w), is 

 again formed ; and a cleavage-cavity (s) is formed between 

 this germ-membrane {Blastoderma, Fig. 47, h) and the 

 nutritive yelk. After this the thickened, swollen edge 

 turns inward, and a simple layer of larger, darker-coloured 

 cells grows from the edge, centripetally towards the middle 

 of the cleavage-cavity (Fig. 48). The meeting of these two 

 edges at a central point gives rise to the intestinal layer, or 

 entoderm (Fig. 48, i). This attaches itself immediately to 

 the roof of the cleavage-cavity, the cells of which form the 

 skin-layer, or exoderm (Fig. 49, i). This completes the 

 Gastrula of the Chick, a flatly extended, disc-shaped Gas- 

 trula (Discogastrula), resembling that of the Osseous Fish 



