3$ 



CHAPTER II 



The Mechanism of Sex-determination 



In many species of animals and plants two kinds of 

 individuals are produced in every generation. This 

 process occurs with such regularity and persistence that 

 our minds naturally seek some mechanism, some sort 

 of orderly machinery, by which this condition is brought 

 about. Yet from the time of Aristotle almost to the 

 present day the problem has baffled completely all 

 attempts at its solution. However, the solution is very 

 simple. Now that we hold the situation in our grasp, 

 it seems surprising that no one was keen enough to 

 deduce it by purely theoretical reasoning. At least 

 the general principles involved might have been de- 

 duced, although we can see that without an intimate 

 knowledge of the changes that take place in the germ- 

 cells the actual mechanism could never have been 

 foretold. 



The bodies of animals and plants are composed of 

 millions of protoplasm-filled compartments that are 

 called cells. In the middle of each cell there is a sphere, 

 or nucleus, containing filaments called chromosomes 

 (Fig. 5). 



At each division of a cell the wall of the nucleus is 

 absorbed, and the thread-like chromosomes contract 

 into rod-shaped, or rounded bodies (Fig. 6). Each 

 chromosome splits lengthwise into halves ; the halves 



35 



