REPRODUCTION 



205 



cells. They possess the same number of chromosomes 

 and divide after the usual karyokinetic changes homo- 

 type mitosis. 



Germinal cell or primary spermatocyte at rest. 



Primary spermatocyte in process of maturation, show- 

 ing four chromosomes, two (P and p) of paternal and 

 two (M and m) of materna' origin. 



Conjugation of the chromosomes. Each pair consists 

 of a paternal and a maternal chromosome. The ap- 

 pearance, of two tetrad-like formations, is easily mis- 

 interpreted to mean that at this stage the cell has 

 eight, or twice the somatic number of chromosomes. 

 Or, if each pair happens to be mistaken for a single 

 chromosome, one-half of the usual nnmber. 



When the conjoined chromosomes separate they do not 

 undergo the longitudinal splitting characterizing 

 homotype mitosis, in which one-half of each chromo- 

 some goes to an opposite pole of the cell, but whole 

 chromosomes now move to opposite poles hetero- 

 type mitosis. 



Thus arise the secondary spennatocytes, each with two 

 or one-half the reduced number of chromosomes. 



From these secondary spermatocytes the spermatozoa 

 or male gametes are formed by homotype mitosis, 

 that is, the chromosomes arrange themselves equa- 

 torially, divide longitudinally, and one-half of each 

 goes to each pole of the cell. 



The process eventuates in mammals in four gametes 

 or spermatozoa of equal value, each with the reduced 

 number two of chromosomes. 



By subsequent modifications of shape these become the 

 well-recognized spermatozoa. 



Fis. 81. Diagram explaining the maturation of "the male germinal cells and the 

 formation of the male gametes or spermatozoa spermatogenesis. This process in- 

 variably consists of two cell divisions, one immediately following the other. The first 

 is invariably, by a peculiar modification of the mitotic changes, known as heterotype 

 mitosis; the second, by the usual or homotype mitosis. As it is more easy to repre- 

 sent the changes diagrammatically where the chromosomes are few, a cell with four 

 chromosomes is chosen, and as it is more easy to describe the process where the 

 number of resulting spermatozoa is small, as in mammals, where there are always 

 four, than among others where there may be great numbers of spermatozoa, it will 

 be assumed that it is the germinal cell of a mammal that is under consideration. 



When the time of functional activity arrives these cells 

 which in the higher animals are known as primary 



