60 SPERMATOGENESIS [CH. 



some Protozoa and in the higher plants) several or many 

 cell-generations earlier. The chromosome reduction is 

 brought about by a nuclear division of a peculiar type, of 

 which the essential feature is that instead of splitting longi- 

 tudinally as in a normal division, the chromosomes become 

 associated in pairs and then separate entire, one chromosome 

 of each pair going to each pole of the spindle. That this is 

 the true nature of the process is now generally recognised, 

 although there is no universal agreement about the details 

 by which it is effected, and until comparatively recently 

 there was extreme divergence of opinion with regard to the 

 interpretation of the phenomena. Gradually, however, 

 agreement as to the fundamental similarity of apparently 

 different methods is being reached, and the former acute 

 conflict of opinion is being replaced by something approach- 

 ing unanimity, at least as far as the basal principles are 

 concerned. The process of reduction is most easily studied 

 in the development of the spermatozoa of animals and of 

 the pollen grains of the flowering plants, and as a general 

 introduction to it, it will be convenient to give a somewhat 

 generalised account of spermatogenesis. 



In the majority of animals both spermatozoa and ova 

 are descended from cells which appear rather early in 

 development and are known as primitive germ-cells. These 

 cells, usually together with a certain amount of other tissue 

 which becomes associated with them, give origin to the 

 gonads (testes and ovaries), and very frequently they give 

 rise not only to the germ-cells themselves but also to 

 accessory cells of various sorts. As the gonad becomes 

 completely formed, cells derived from the primitive germ- 

 cells can be distinguished as those from which the actual 

 gametes (spermatozoa or ova) will be produced, and these 

 are known in the male as spermatogonia, in the female as 



