CHAPTER VI 



TWINS IN RELATION TO GENERAL BIOLOGICAL 



PROBLEMS 



The study of twins throws light especially on the 

 following problems: (i) the time of and the mechanics 

 of sex-determination; (2) the significance of sex-ratios; 

 (3) the mechanism of sex-differentiation; (4) is twin- 

 ning hereditary? (5) the modes of inheritance in 

 monozygotic or polyembryonic twins; (6) the nature 

 and significance of symmetry reversals in monozygotic 

 twins. The first five problems are discussed in the 

 present chapter, while the last two problems are reserved 

 for the next chapter. 



TWINS AND THE PROBLEM OF SEX-DETERMINATION 



The problems of sex are today attracting the widest 

 attention, and among these problems that of the mechan- 

 ism of sex-determination appears to have been largely 

 solved. It appears that in a vast number of animals 

 of all grades of organization, from worms to man, sex 

 is determined at the time of fertilization. In some 

 forms sex is determined in the egg, for there are two 

 distinct types of eggs, male-producing and female- 

 producing. In other cases the eggs are all alike and 

 produce females if allowed to develop partheno- 

 genetically (without fertilization), but produce half 

 males and half females if fertilized, the result being due 



no 



