REPRODUCTIVE ORGANS IN THE MALE. 281 



age of forty-five. The explanation offered to ac- 

 count for this difference in the sexes is sought in the 

 blood supply to the generative organs. There is a 

 decrease in the nourishment to the ovaries as the 

 menopause approaches, due to a contraction of the 

 blood-vessels that supply the organ. The testes, on 

 the other hand, have a liberal supply of blood 

 throughout life. 



The development of spermatozoa has been re- 

 corded with great accuracy, particularly in ascaris, 

 insects, amphibians, and fishes, and there is little 

 doubt but that the processes in mammalia are es- 

 sentially the same. 



The spermatogonia cells of the testicle after repeated 

 division produce what are called primary spermato- 

 cytes. Each of the latter divides by somatic mito- 

 sis to produce two secondary spermatocytes, and these 

 divide by reduction mitosis to produce spermatids, 

 which, in turn, are moulded into individual sperma- 

 tozoa. From every primary spermatocyte, there- 

 fore, four spermatozoa ultimately ripen, a detailed 

 account of which will now be considered. 



Multiplication of spermatogonia does not differ 

 from other somatic mitosis. The primary sperma- 

 tocytes, however, show a more condensed form of 

 chromatin, and while the usual spireme structures 

 develop, the chromosomes pair and fuse. This is not 

 a chance fusion, but is said to be a selective union 

 called synapsis of chromosomes. This coalition ap- 

 parently reduces the chromosomes to one-half the 

 original number. The synapsis is usually a collateral 

 one, sometimes end to end, and while in most instances 

 the union is complete, in other cases the double na- 



