226 THE EVOLUTION OF MAN. 



an eiglit-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 otlier 

 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 Qgg is, however, thinner and 

 flatter than that of the Qgg of the Osseous Fish (Plate III. 

 Fig. 21). 



In the Hen's Qgg, 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 {Blastoderm a, 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 tliese 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 



