EGG-CLEAVAGE. 1 89 



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.^^ 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 11. Fig. S.^^) 



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

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

 germ-membrane-cells (celhdce hlastoderTnicce)-, 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 hlastodermica).^^ The inner cavity of the ball, 

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

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

 (hlastocoeloma). 



