THE CELL. 



Membrane of 

 ovum. 



Nucleus of 



ovum. 

 Spermatozoon 



entering. 



Protoplasm of 

 ovum with 

 deutoplastic 

 granules. 



Fig. 31- 



Female 

 pronu- 

 cleus. 



Head of 

 spermato- 

 zoon with 

 centro- 

 some. 



Female 

 pronu- 

 cleus. 



Male 



pronu- 

 cleus. 



Fig- 32- Fig. 33. 



Figs. 31-33. Diagrams of the process of fertilization, after Boveri. 

 Figure 31, the ovum is surrounded by spermatozoa, one of which is in the act of 

 penetration. Toward it the yolk is pushed forward in a short, rounded process. Figure 

 32, the tail of the spermatozoon has disappeared. L>eside the head is a centrosome with 

 polar radiation. Figure 33, the pronuclei approach each other. 



ovum matures the germinal vesicle approaches the periphery, and a 

 peculiar metamorphosis, which may be regarded as a double, un- 

 equal division of the egg-cell, takes place. One portion, in the case 

 of both divisions, is much smaller than the other, and is known as 

 a polar body. At the close of these divisions, during which the 

 chromosomes have been reduced to half the original number, there 

 are, therefore, two polar bodies and the matured ovum, which is 

 now ready for impregnation. 



The development of the male sexual cell in its earlier stages is sim- 

 ilar to that of the ovum. They are derived from cells known as spcr- 

 matogoncs. These divide into equal parts, forming the cells of a 

 second generation, the spermatocytcs. From a further division of the 

 spermatocytes, during which division the chromosomes are reduced 

 to half the number, the spermatids are produced. These latter are 

 then changed directly into spermatozoa. The reduction division of 

 the egg-cell and that of the spermatocytes is in principle the same, 

 except that in spermatogenesis all cells become matured sexual cells 



