62 



MANUAL OF ZOOLOGY 



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) 



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m 



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fe 



ill 



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Fig. 27. — Various stages i 



the segmentation of the oosperm. (From Gegenbaur's 

 Co}nparativc Anatomy.) 



in the same manner into two — four cells being thus formed; 

 the four divide to form eight, the eight to form sixteen, 

 and so on ; until, by this process of division and sub- 

 division, 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. 



