76 BIOLOGY AND SOCIAL PROBLEMS 



tined to give rise to one individual divides in 

 the case of identical twins into two. Therefore 

 the subsequent history of the embryo, such as 

 the position afforded it in the mother's body or 

 the character and amount of nourishment pro- 

 vided for it, can have no influence on its sex. 

 This interpretation of the condition in man 

 is abundantly supported by recent work on 

 one of the primitive mammals, the armadillo. 

 It was long ago known that all the young in 

 a single litter of this animal were of the same 

 sex. Newman and Patterson have recently 

 shown that in the armadillo only one egg is 

 discharged from the ovary at each period of 

 heat, and that, if this egg is fertilized, four 

 embryos usually result, all of the same sex. 

 These four embryos, to use Morgan's state- 

 ment of the case, are more like each other 

 than like the embryos of any other litter, or 

 even more like each other than they are like 

 their own mother. In other words they are 

 identical quadruplets exactly comparable to 

 identical twins in man. The fact that they are 

 always of the same sex and are positively 

 known to come from one egg cell supports 

 most completely the conclusion arrived at 

 from the study of human twins. 



