128 TEE POPULAR SCIENCE MONTHLY 



tional egg while the other three are small rudimentary eggs which are 

 called polar bodies and which take no further part in development 

 (Fig. 23, D-F). The fertilization of the egg usually takes place coin- 

 cidently with the formation of the polar bodies — and so we come back 

 once more to the stage from which we started, thus completing the life 

 cycle. 



4. Sex Determination 



In the formation of the sex cells one can frequently distinguish at 

 an early stage, differences between the larger oogonia and the smaller 

 and more numerous 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 at a still 

 earlier period of development. 



The cause of sex has been a favorite subject of speculation for thou- 

 sands of years. Hundreds of hypotheses have been advanced to explain 

 this perennially interesting phenomenon. The causes of sex determina- 

 tion 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 generally founded upon a few observa- 

 tions 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 

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

 war or peace or education on the relative number 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. 



This was the condition regarding the causes of sex determination 

 which prevailed up to the year 1902. Immediately preceding that year 

 it had been found that the kinds of spermatozoa were formed in equal 

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

 "accessory" 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. 34 B). 

 Later, in the reduction division, when the synaptic pairs separated, the 

 odd chromosome went entire into one of the daughter cells, and the 

 spermatozoa formed from this cell contained one chromosome more 

 than those formed from the other daughter cell (Fig. 34 C and D). 



