264 



EVOLUTION AND ANIMAL LIFE 



The nucleus, or germinal vesicle, is large, and contains a net- 

 work of chromatin together with one or more conspicuous 

 nucleoli. 



There are three periods usually recognized in the develop- 

 ment of the egg cell, viz.: (1) The period of multiplication; (2) 

 the period of growth; and (3) the period of maturation. The 

 first period is characterized by a continued series of divisions 

 of the primitive reproductive cell and its descendants, which 



A 



FIG. 149. Formation of the chromatins and tetrads in the spermatogenesis of Ascaris 

 megalocephala. In A-C, the whole cell is shown; in D-H, only the nucleus. (After 

 Brauer.) 



produces a large number of "ovogonia." Succeeding this is 

 a period of growth in which the ovogonia increase greatly in 

 size, mainly through the production and storing up of food 

 yolk. At the close of this period the germ cell, now termed a 

 "primary ovocyte," enters upon the maturation period, in 

 which it undergoes two divisions in rapid succession, by means 

 of which two minute cells, the polar bodies, are cut off from 

 the egg. Through these two divisions the number of chromo- 

 somes in the egg nucleus is reduced to one half that which is 

 found in the other cells of the body. The first polar body also 

 usually divides, and thus, at the close of the period of matura- 

 tion, four cells result: one large mature egg cell, ready for the 

 fertilization which initiates the development of the embryo, 

 and three minute polar bodies, which are to be regarded simply 

 as rudimentary eggs. The nuclei of these four cells are exactly 



