228 GENERAL ZOOLOGY 



divisions. During the first maturation division the oocyte 

 divides into two cells of very unequal sizes, the larger called the 

 secondary oocyte and the smaller the first polar body or polocyte. 

 In the second maturation division the secondary oocyte divides 

 into two cells, again of unequal size, the larger of which is the 

 ootid and the smaller the second polar body. At the same time 

 the first polar body may divide to form the third and fourth polar 

 bodies. As a result of the two maturation divisions a primary 

 oocyte may produce four cells: a single ootid, which is relatively 

 very large and is the mature ovum, and three small polar bodies. 

 The polar bodies may remain for a time attached to the egg 

 at the point where they are extruded, but sooner or later they are 

 lost and play no further part in development of the egg. It is 

 generally supposed that the polar bodies have the same poten- 

 tialities as ova but lack sufficient cytoplasm and yolk for develop- 

 ment. This view is supported by experiments in which it was 

 found that (in certain flatworms) abnormally large first polar 

 bodies, when fertilized, develop to an early larval stage. Devel- 

 opment up to a certain point is possible in this case because the 

 polocyte contains a certain amount of yolk. In most animals it 

 is very small and consists of little more than a nucleus (Fig. 

 141). ' 



Spermatogenesis. — In the male, the division period produces a 

 large number of spermatogonia. These then pass through a 

 growth period, but the primary spermatocytes formed as a result 

 are much smaller in size than the corresponding primary oocytes 

 of oogenesis (Fig. 139), though greater in number. In the first 

 maturation division each primary spermatocyte produces two 

 secondary spermatocytes of equal size. Likewise, in the second 

 maturation division each secondary spermatocyte divides to 

 form two spermatids of equal size. Therefore each primary 

 spermatocyte produces four spermatids. Each spermatid now 

 undergoes a further development, called spermiogenesis, as a 

 result of which it is transformed into a spermatozoon. In this 

 transformation the nucleus of the spermatid is crowded into 

 what is called the head of the spermatozoon, while the remaining 

 parts of the spermatozoon, called the middle piece and tail, are 

 formed of the cytoplasm. The tail is an organ of locomotion by 

 means of which the spermatozoon propels itself through fluid. 

 The spermatozoa of some animals lack a tail and are nonmotile. 



