62 SPERMATOGENESIS [CH. 



divide and begin to increase considerably in size. At this 

 stage they are called primary spermatocytes in the male and 

 oocytes in the female. From this point onwards the be- 

 haviour in the two sexes is different, and for the present 

 only the male will be considered. At the close of the growth- 

 phase of the primary spermatocytes or, as they are some- 

 times called, spermatocytes of the first order, important 

 changes occur in the nucleus, which will be discussed in 

 detail below; they result, however, in the production of 

 chromosomes each of which is double, with the consequence 

 that the nucleus contains, instead of the somatic number of 

 single chromosomes, half the somatic number of dciible_ 

 chromosomes. The primary spermatocytes now divide in 

 such a way that the components of the double chromosomes 

 separate from one another, in consequence of which each of 

 the daughter cells, known as secondary spermatocytes , con- 

 tains half the somatic number of single chromosomes. 

 These secondary spermatocytes then divide again immedi- 

 ately by a typical mitotic division, in which the chromo- 

 somes split longitudinally; the cells so produced are called 

 spermatids and also contain half the somatic number of 

 chromosomes. The spermatids are then converted directly 

 into spermatozoa without any further division. 



The whole process may be summarised thus. The sper- 

 matogonia contain the somatic number of chromosomes; 

 each spermatogonium of the final generation enters on a 

 period of growth and nuclear change and becomes a primary 

 spermatocyte. The primary spermatocyte just before divi- 

 sion contains half the somatic number of double, chromo- 

 somes; it divides by a peculiar division giving rise to two 

 secondary spermatocytes which have half the somatic num- 

 ber of single chromosomes* The two secondary spermato- 

 cytes so formed divide by a typical division, giving rise to 



