62 



MANUAL OF ZOOLOGY 



SECT. 



substance of the protoplasm becomes cleft into two parts 

 (Fig. 27), each half containing one of the nuclei, so that 

 two complete cells result. This process, it will be observed, 

 is essentially the same as the binary fission of Amoeba 

 and other Protozoa : in the Metazoan, however, the two 

 cells do not become separated from one another as the 

 two parts of the divided Amoeba do, but remain in contact 

 and undergo further changes. Each of them divides (Fig. 

 27) in the same manner into two four cells being thus 



FIG. 27. Various stages in the segmentation of the oosperm. 

 Comparative Anatomy.) 



(From Gegenbaur's 



formed ; the four divide to form eight, the eight to form 

 sixteen, and so on, until, by this process of division and 

 subdivision, the oosperm becomes segmented into a large 

 number of comparatively small cells. In this mass of cells 

 an arrangement into layers, the germinal layers, becomes by 

 and by discernible ; and from these layers of cells are 

 developed eventually all the parts of the body of the 

 Metazoan. 



