236 HEREDITY AND SEX 



obviously may only mean that more of the large eggs 

 are likely to be laid at one temperature than at another. 

 In fact, temperature seemed to act so promptly accord- 

 ing to Von Malsen's observations that it is very un- 

 likely that it could have had any influence in deter- 

 mining the kind of egg produced, but rather the kind 

 of egg that was more likely to be laid. We may dis- 

 miss this case also, I believe, as not showing that sex 

 is determined by temperature. 



SEX-DETERMINATION IN MAN 



Let us now proceed to examine the evidence that 

 bears on the determination of sex in man. I shall 

 draw on three sources of evidence : 



1. Double embryos and identical twins. 



2. Sex-linked inheritance in man. 



3. Direct observations on the chromosomes. 



The familiar case of the Siamese twins is an example 

 of two individuals organically united. A large series 

 of such dual forms is known to pathologists. There 

 are hundreds of recorded cases. In all of these both 

 individuals are of the same sex, i.e. both are males 

 or both are females. There is good evidence to show 

 that these double types have come from a single fer- 

 tilized egg. They are united in various degrees (Fig. 

 110) ; only those that have a small connecting region 

 are capable of living. These cases lead directly to 

 the formation of separate individuals, the so-called 

 identical twins. 



Galton was one of the first, if not the first, to recognize 

 that there are two kinds of twins — identical twins and 

 ordinary or fraternal twins. 



