OPERATION OF THE GENETIC SYSTEM 67 



of the male the result is that produced by the interaction of 

 but one such center with the other contents. 



The result of this difference in balance of the chromo- 

 somes is in fact to produce the very great differences, struc- 

 tural, physiological, mental, that distinguish the female 

 from the male. The fact that so many and so great differ- 

 ences, of so many diverse kinds, are brought about without 

 any original difference in the kinds of materials present, but 

 only in balance, only in the number of centers of growth 

 for certain materials, is of the greatest significance for the 

 nature of the processes of development; it should never be 

 lost sight of in considering the facts of heredity. Other dif- 

 ferences produced by diversity of chromosomes, to be taken 

 up later, may perhaps be brought about in a similar way. 



Through What Means Do the Chromosomes Produce 



Sex Differences? 



How can differences between chromosomes produce such 

 great differences as those shown by individuals of different 

 sexes? What is their method of action in bringing about 

 sex differences? 



Although a complete answer to these questions cannot be 

 given, much is known that bears on them. Many things have 

 been discovered as to the means through which chromosome 

 differences produce sex differences, and these things are 

 most instructive for forming a conception of the nature of 

 development and heredity. 



Activity of the Chromosomes in Development 



As before seen, the chromosomes are very active bodies. 

 They are continuously working at the cytoplasm of the 

 cells, interacting with it, changing it. In Chapter II this was 

 described. In the very young cells, just after cell division, 

 the chromosomes are small compact bodies. They begin to 



