158 Heredity and Environment 



fifth week, and it is evident that the real causes of this differ- 

 ence 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. Hundreds of hypotheses have been advanced 

 to explain this perennially interesting phenomenon. 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 determined. Unfortunately these hypotheses and rules are gen- 

 erally founded upon a few observations 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 forgets the failures 

 of a rule and remembers the times when it holds good it is possi- 

 ble to believe in the influence of food or temperature or age, of 

 war or peace or education on the relative numbers of the sexes, 

 or on almost any other thing. By statistics it has been shown 

 that each of these things influences the sex ratio, and by more 

 extensive statistics it has been proved that they do not. 



i. Chromosomal Determination: XO Type. This was the 

 condition regarding the causes of sex determination which pre- 

 vailed up to the year 1902. Immediately preceding that year it 

 had been found that two kinds of spermatozoa were formed in 

 equal numbers in certain insects; one of these kinds contained a 

 peculiar "accessary" or "odd" chromosome, and the other lacked 

 it. The manner in which these two types of spermatozoa were 

 formed had been carefully worked out by several investigators 

 without any suspicion of the real significance of the facts. It 

 was shown that an uneven number of chromosomes might be 

 present in the spermatogonia of certain insects and that when 

 maternal and paternal chromosomes united in pairs in synapsis 

 one "odd" chromosome was left without a mate (Fig. 51 B). 

 Later, in the reduction division, when the synaptic pairs sep- 

 arated, the odd chromosome werit entire into one of the daughter 

 cells, and the spermatozoa formed from this cell contained one 



