40 HEREDITY AND SEX 



similar reduction occurs in the sperm, excepting that no 

 chromatin is lost, but is redistributed amongst four 

 spermatozoa. Egg and sperm-nucleus each have in 

 consequence the haploid or half number. By combin- 

 ing they bring up the number to that characteristic of 

 the species. 



The history of the germ-cells, that we have just 

 traced, is the background of our knowledge of the pro- 

 cess of heredity in so far as observable changes in the 

 germ-cells have been made out. We owe to Weismann 

 more than to any other biologist the realization of 

 the importance of these changes. It is true that 

 Weismann contributed only a part of the actual facts 

 on which the interpretation rests. Many workers, 

 and a few leaders, have laboriously made out the com- 

 plete account. But Weismann, by pointing out the 

 supreme importance of the changes that take place at 

 this time, has furnished a stimulus that has acted like 

 yeast in the minds of less imaginative workers. 



We are now in a position to apply this knowledge to 

 the interpretation of the mechanism by means of which 

 sex is determined. 



THE CYTOLOGICAL EVIDENCE 



If we study by means of modern histological methods 

 the body cells of the male of the insect, Protenor 

 belfragei, we find, when each cell is about to divide, 

 that a group of chromosomes appears like that shown 

 in Fig. 21, A. There are twelve ordinary oval chromo- 

 somes, and one much larger than the rest. This group 

 of chromosomes is characteristic of all divisions of the 

 cells of the body, regardless of whether the cells belong 



