206 BIOLOGY: GENERAL AND MEDICAL 



spermatocytes, manifest certain peculiar proliferative 

 activities, the chief of which is known as the reduction 

 division. The chromatic substance in the nucleus gathers 

 together to form the usual number of chromosomes, but in- 

 stead of assuming their customary appearance and arrange- 

 ment they appear in pairs or gemmini. This conjugation 

 of the chromosomes leads to an appearance easily misin- 

 terpreted to mean that the cell has either twice the usual 

 number, or only half of the usual number. The appearance 

 of the nuclear spindle is now followed by the separation of 

 the gemmini, and the passage of each component chromo- 

 some to an opposite pole, so that the resulting two cells 

 secondary spermatocytes receive whole chromosomes, and 

 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. 



