CHAPTER V 



The Maturation of the Germ-cells : Spermatogenesis 



THE two preceding chapters have dealt with the phe- 

 nomena of cell-division in ordinary body cells. In 

 these cells the essential feature of nuclear division is the 

 accurate halving of the chromosomes^with the result tjiateach 

 of the two daughter-nuclei contains the same number as the 

 parent nucleus from which they arose, and a corollary from 

 this is that every nucleus of the body (except where an 

 abnormal division has occurred) contains a similar com- 

 plement of chromosomes. It is evident, however, that at 

 some point a different process must take place, for in almost 

 all animals and in the great majority of plants at some 

 stage in the life-cycle sexual reproduction occurs. The 

 essence of sexual reproduction is the union of two cells 

 derived from different parents, accompaniecTEy the fusion 

 of their nuclei, and when conjugation of nuclei occurs it is 

 clear that the chromosome number of the zygote (product 

 of the union) must be doubled. If, then, the germ-cells of 

 the two parents each contained the number of chromosomes 

 typical of the species, the number in the zygote would be 

 doubled, and, unless there were some provision for halving 

 the number, the doubling would recur in every sexually- 

 produced generation. 



In all organisms in which sexual reproduction occurs this 

 difficulty is avoided by the process known as the reduction 

 of the chromosomes, which usually takes place in the cell- 

 divisions immediately preceding the production of the con- 

 jugating cells or gametes, although it may occur (e.g. in 



