2l8 THE EVOLUTION OF MAN. 



small cell alone is the formative yelk, and is the only pan 

 concerned in the cleavage process, which does not extend 

 to the nutritive yelk.'^*' 



The cleavage of the parent-cell, or the formative yelk, 

 proceeds entirely independently of the nutritive yelk, and 

 in quiet, regular, geometric progression. (Cf. Plate III. Fig. 

 18-24.) Only the formative yelk, with the contiguous 

 portion of the nutritive yelk (n), is represented in the 

 perpendicular section (through a meridian-plane); the 

 greater part of the nutritive yelk and the egg-membrane 

 is therefore omitted. The parent-cell (Fig. 18), first sepa- 

 rates into two similar cleavage-cells (Fig. 19). By repeated 

 division, this gives rise to 4, then 8, then 16 cells (Fig. 20). 

 By continued contemporaneous division, 32, and then 64 

 cells originate from these ; and so the process goes on. All 

 these cleavage-cells are alike in size and character. At last 

 they form a lentil-shaped mass of closely layered cells (Plate 

 III. Fig. 21). This entirely corresponds to the globular 

 mulberry-germ of the primordial cleavage-process (Morula, 

 Plate II. Fig. 3). The cells of this lentil-shaped mulberry- 

 germ now move oflf in a peculiar centrifugal direction, 

 so that the mulberry-germ changes into a vesicular germ 

 {Blastula, Plate III. Fig. 22). The ordinary lentil be- 

 comes a disc, in the shape of a watch-glass, with thickened 

 edges. Just as a watch-glass lies upon a watch, this con- 

 vex cellular disc lies on the upper, more slightly arched, 

 pole surface of the nutritive yelk. Meanwhile, liquid has 

 collected between the disc and the surface of the nutritive 

 yelk, so that a low circular cavity has been formed (Fig. 22, s). 

 This is the cleavage-cavity, and corresponds to the cleavage- 

 cavity in the centre of the pr.Ungentic Blastula (Plate II. 



