68 GENETICS 



absorb material from the cytoplasm around them. They 

 thus enlarge, becoming vesicles filled with substance. Each be- 

 comes doubtless a hundred times as large as before. In time 

 the vesicles become so large that they are crowded together 

 to form the large nucleus (figure 15). 



What becomes of this large amount of fluid taken in by 

 the chromosomes? It undoubtedly becomes chemically 

 changed by interaction with the chromosomes. There is 

 evidence of this in the chemical reactions of the material of 

 the nucleus as compared with those of the cytoplasm, and in 

 certain other processes that will be discussed later. 



The nucleus thus has filled itself with cytoplasm, which 

 has been acted on, changed, by the chromosomes. Then, be- 

 fore the next cell division, it pours this altered material 

 back into the cytoplasm. The outside membrane of the nu- 

 cleus dissolves and all the materials pass out, except that 

 there remains still a small concentrated chromosome, from 

 each one of the former vesicles. 



This taking in of cytoplasm, altering it, and giving it off 

 again. Is repeated at every cell division, so that It is con- 

 tinuously going on throughout development; it is occurring 

 in every cell of the body. In this way the cytoplasm becomes 

 gradually changed. Different kinds of materials are pro- 

 duced In the different cells; the different tissues and organs 

 of the body are thus ultimately formed. 



Thus it might be expected that sex differences would be 

 produced through the fact that different chemical reactions 

 occur when the cells contain but one X, from those that take 

 place when two X's are at work. And there is proof that this 

 is true. To illustrate these matters, we may summarize what 

 happens in the development of sex differences in mammals. 



( I ) In mammals the two sexes differ at the beginning of 

 their lives. In the fertilized egg, as to the number of X- 

 chromosomes they contain. Both contain the usual two sets 



