THE 



POPULAR SCIENCE 



MONTHLY. 



DEOEMBEE, 1903. 



RECENT THEORIES IN REGARD TO THE DETERMINA- 

 TION OF SEX. 



By Professor T. H. MORGAN, 



BRYN MAWR COLLEGE. 



TT was long believed that the sex of the embryo is determined at a 

 -■- relatively late stage in its development, and therefore it seemed 

 probable that external factors must decide whether the embryo is to 

 become a male or a female individual. Many views have been held 

 as to what these external factors are, and from time to time hopes 

 have been held out that it might be possible to regulate, by artificial 

 means, the sex of the developing embryo. 



In the last few years opinion has begun to turn in the opposite 

 direction, and several attempts have been made to prove that the sex 

 of the embryo is determined in the egg. That this must be the case 

 in man seemed to be indicated by the fact that 'identical twins' are 

 always of the same sex. There can be little doubt that such twins come 

 from the same egg, and the presumption is strongly in favor of the 

 view that they represent the separated first two cells of the segmenting 

 egg. These twin embryos are enclosed in the same chorion, which 

 further indicates that they have come from one egg. The 'ordinary 

 twins' of man are no more like each other than are any other two 

 children born at different times. The pair of ordinary twins often 

 consists of a male and a female. Since the embryos that give rise to 

 ordinary twins are subjected to practically the same conditions during 

 their uterine life, and are often, as has been said, a male and a female 

 in a pair, it follows that in man the external conditions that affect the 

 egg, after it has left the ovary or after it has been fertilized, do not 

 determine the sex. A similar and even more remarkable fact is known 



VOL. LXIV. — 7. 



