196 



PRINCIPLES OF ANIMAL BIOLOGY 



By a transformation in shape, the spermatid becomes a mature sfer- 

 Tnatozoon. This cell consists usually of a head and a whiplike tail, but 

 the forms are very different in different animals (Fig. 1G6). The chromo- 

 somes are all contained in the head, the tail being merely a motile organ. 



Oogenesis. — The ripening of the female germ cells is in most respects 

 similar to that of the male. The early germ cells or oogonia undergo a 

 period of multiplication in which they divide by ordinary mitosis. 

 Eventually this ordinary division ceases, and the cells are ready to initiate 



Q 



B 



I 



Fig. 166. — Different forms of .spermatozoa: A, badger; B, slieldrake; C, sturgeon; 

 D, flycatcher; E, opossum; F, lobster; G, crustacean Polyphemus; H, crab Droniia; /, crab 

 Porcellana; J , crustacean Ethusa. (A-D after Ballowitz; F after Her rick; G after Zacharias; 

 H-J after Grobben. From Wilsori, " The Cell in Development and Heredity." Courtesy of 

 The Macmillan Company.) 



the maturation process. They are now known as primary oocytcfi. 

 These oocytes grow rapidly to many times their original volume, the 

 growth being much greater than in the male. 



During growth the chromosomes meet in pairs, each pair, as in the 

 male, being composed of one maternal and one i)aternal chromosome. 

 Each chromosome may divide or be duplicated as they come together, so 

 that the pair presents a quadruple body, the tetrad. 



Divisions in Oogenesis. — These tetrads are divided in the remainder 

 of the process, first into dyads, next into their single components, in a 

 manner strictly comparable to the divisions in the male. When a sjjindle 

 is formed for the first division, it appears not in the center but near the 



