EGG-CLEAVAGE. 189 



divides into two similar halves, the division of the cell- 

 kernel again preceding that of the surrounding proto- 

 plasm. The eight cleavage-cells thus produced bisect in 

 the same way into sixteen. Thirty-two cleavage cells are 

 formed from these by further division. As each of these 

 again bisects, sixty-four of these cells are produced ; after- 

 wards one hundred and twenty-eight, and so on. 59 These 

 repeated and similar bisections finally result in the produc- 

 tion of a globular mass of similar cleavage-cells; we call 

 this mass the mulberry-germ (morula). The cells lie as 

 close together as the drupes of a mulberry or blackberry ; 

 .so that the entire surface of the round mass appears rugged 

 (Fig. 22, E}. . (Cf. Plate II. Fig. 3. 60 ) 



After this egg-cleavage is completed, the solid mulberry- 

 germ changes into a hollow globular vesicle. A watery 

 liquid or jelly collects in the centre of the solid ball ; the 

 cleavage-cells part asunder, and all seek the surface of the 

 ball. Here by mutual pressure they become multilaterally 

 flattened, assume the form of truncated pyramids, and range 

 themselves in order; side by side, in a single stratum 

 (Fig. 22, F, G}. This cell-stratum is called the germ-mem- 

 brane (HastoderincC) ; the cells (all of one kind), a simple 

 stratum of which forms the germ-membrane, are called the 

 germ-membrane-cells (cellulcv blastodermicce) ; and the entire 

 hollow ball, the walls of which are composed of these cells, 

 is called the germ-membrane-vesicle, or, briefly, the germ- 

 vesicle, or vesicular-germ (blastula ; formerly called the 

 vesicula blastodermica)* 1 The inner cavity of the ball, 

 which is filled with clear liquid or jelly, is called the 

 cleavage-cavity (cavum segmentarium), or the germ-cavity 

 (blastocoeloma). 



