194 BIOLOGY: GENERAL AND MEDICAL 



only half of the usual number. This is the reduction di- 

 vision, and is described as heterotype mitosis. After a 

 short interval each secondary spermatocyte again divides 

 into a number of small cells spermatozoa varying 

 among different animals. This division is effected by 

 the homotype mitosis, each resulting cell receiving parts 

 of chromosomes, but always the reduced number. 



In man and probably most, if not all, mammals the 

 primary spermatocyte, with the full somatic number of 

 chromosomes, gives rise, by the heterotype mitosis, to two 

 secondary spermatocytes with the reduced number, and 

 each of these, by homotype mitosis, to two spermatozoa, 

 each with the reduced number. The four spermatozoa 

 (gametes) are of uniform size, similar in appearance, and, 

 so far as is known, of equal functional value. 



Oogenesis. The germinal cells of the female are sim- 

 ilarly set aside in the gonads ovaries and, like those of 

 the male, undergo multiplication during the period of 

 growth and development by the usual karyokinetic 

 changes homotype mitosis. After the perfected develop- 

 ment of the higher organisms mammals they seem to 

 undergo no further increase, but remain inactive until 

 sexual perfection and its various activities arouse them 

 to certain changes described as maturation. The cells 

 mature one by one, and prepare for fertilization by a series 

 of mitotic changes analogous to those of the opposite sex. 



The beginning of the maturation is shown by preparation 

 for the reduction division. The chromatic substance 

 gathers into the usual number of chromosomes, which 

 form gemmini and take their usual position in the nuclear 

 spindle. The gemmini then separate, each gemminus 

 turning to the pole of the cell opposite to its fellow. The 

 division, therefore, ends in the appearance of two cells, 

 each with one-half the somatic number of chromosomes. 



These cells, unlike the secondary spermatocytes, are not 

 of uniform size. One is very large, the other very small. 

 The reduction division is immediately followed by division 

 of both of the newly formed cells, but this time by homo- 



