THE DETERMINATION OF SEX 



gards the freshness or staleness of the egg as the im- 

 portant factor in predetermining sex. According to 

 this idea it is thought that an egg shortly after ovula- 

 tion tends to produce a female, while one that remains 

 some time in the oviduct tends to produce a male. 



The idea that two distinct types of eggs are formed 

 is not altogether new. Thus, entirely without biological 

 foundation, the theory has been propounded that one 

 ovary gives rise to male-producing eggs and the other 

 forms female-producing eggs. Equally without founda- 

 tion is the theory that in one testis male-determining 

 spermatozoa are produced and in the other, female- 

 determining spermatozoa. 



Modern theories of sex determination hold to the 

 first and second of the three possibilities mentioned 

 above. If there are two kinds of eggs, male-producing 

 and female-producing, then the sex of the individual is 

 already fixed at the time of the extrusion of the first 

 polar cell, before the sperm-nucleus has united with the 

 egg-nucleus in fertilization. If there are two kinds of 

 sperm, male-determining and female-determining, then 

 sex depends upon the type of sperm uniting with the 

 ovum, and it may, therefore, be said that sex is deter- 

 mined at the time of fertilization. 



2. SEX CHROMOSOMES 



Our present day stand on sex determination is based 

 entirely on direct observation, both cytological and ex- 

 perimental. In 1902, an unpaired chromosome was ob- 

 served by McClung in the testes of certain Orthoptera. 



