6 THE BIOLOGY OF TWINS 



individuals are produced, affords the most available 

 material for a study of these problems. 



The problems of twinning and those of sex and of 

 heredity are inextricably interwoven; each of them 

 appears to be a corollary of the other. It is not surpris- 

 ing then that some of the most conclusive evidence as 

 to the nature and mode of sex-determination and much 

 new light on the nature and limits of hereditary control 

 come from a study of twins. 



A collection of data upon twinning in mammals 

 brings together more biological curiosities than is fur- 

 nished by any other field of similar scope with which I 

 am acquainted. The armadillos furnish two strangely 

 unique situations quite opposite in character. In Das- 

 ypus there is the sphtting up of a single egg into a 

 number of separate embryos, while in Euphractus two 

 originally separate eggs secondarily undergo extensive 

 fusion of their membranes so as to produce mono- 

 choriaP twins quite deceptively Hke those known to be 

 monozygotic. Twinning in cattle involves the very odd 

 phenomenon of the freemartm, where usually a female 

 born co-twin to a male is sterile and shows certain 

 male characteristics. An analysis of this peculiar situa- 

 tion goes far toward clearing up the problems of the 

 nature of sex and of the factors which control it. 



There are also many strange facts about the va- 

 rious kinds of human twins. DupHcate or identical 

 twins are of unusual interest; conjoined twins of the 



^ Monochorial twins are surrounded by a single chorionic mem- 

 brane. Usually a single chorion covers but one embryo and comes 

 from a single egg; hence it is customarily assumed that the monocho- 

 rial condition implies a monozygotic origin. 



