198 



PRINCIPLES OF ANIMAL BIOLOGY 



The division of the secondary oocyte involves the division of the 

 dyads into their halves. The division of the cytosome is again very 

 unequal, so that one small cell, the second polar body or second polocyte, 

 and one large cell are produced. The large cell, unlike the final cells 

 in the male, does not undergo any change of shape ; its maturation is fin- 

 ished when the second division is completed, and it is therefore a mature 



egg- 



Comparison of Oogenesis and Spermatogenesis. — Comparison of the 



maturation of spermatozoa with that of eggs reveals that with respect to 



Fig. 169. — Homolecithal egg of the sand worm Nereis. C, cytosome; /, fat droplets; 

 m, egg membrane; ri, nucleus; nl, nucleolus; y, yolk spheres. (After Wilson. Courtesy oj 

 The Macmillan Company.) 



the chromosomes the two processes are parallel. The chromosomes unite 

 in pairs and are often at the same time duplicated so as to produce tetrads. 

 Two rapidly succeeding divisions divide the tetrads into dyads and 

 then single chromosomes. 



The final cells contain half as many chromosomes as did the reproduc- 

 tive cell before the process began. These chromosomes may be paternal, 

 or maternal, or paternal and maternal mixed in any pro{)ortion. 



The striking feature in which the processes differ in the two sexes 

 concerns the cytosome. In the female the divisions are very unequal, 

 so that from each original cell there are produced not four functional 

 cells as in the male but only one functional cell and two or three degener- 

 ate ones. 



