THE CELLULAR BASIS 139 



at an early stage differences between the 

 larger oogonia and the smaller and more nu- 

 merous spermatogonia; this difference is the 

 first visible distinction in the development of 

 the two sexes. In the case of the human 

 embryo this distinction can be made as early 

 as the fifth week, and it is evident that the real 

 causes of this difference must be found at a 

 still earlier period of development. 



The cause of sex has been a favorite subject 

 of speculation for thousands of years. Hun- 

 dreds of hypotheses have been advanced to 

 explain this perennially interesting phenome- 

 non. The causes of sex determination have 

 been ascribed to almost every possible external 

 or internal influence and the world is full of 

 people who think they have discovered by 

 personal experience just how sex is deter- 

 mined. Unfortunately these hypotheses and 

 rules are generally foimded upon a few obser- 

 vations of selected cases. Since there are only 

 two sexes the chances are that any hypothesis 

 will be right half the time, and if only one for- 

 gets the failures of a rule and remembers the 

 times when it holds good it is possible to be- 



