THE ORIGIN OF GYNANDROMORPHS. 109 



chromosomes, although this point has not been sufficiently studied to 

 exclude such a process. On the other hand, it has been shown that 

 the growth in question is caused by a sex-linked Mendelian gene that 

 is inherited strictly, as are all Mendelian sex-linked genes. This 

 mutant lethal race of Drosophila arose as a mutation, presumably in 

 the same way as other mutations. If it is not admissible at present 

 to draw any analogy between this case and that of mammalian cancer, 

 it is conceivable at least that mammalian cancer may be due to re- 

 current somatic mutation of some gene. Such a conclusion would, 

 however, not invalidate the view that cancer is more likely to occur 

 in certain families, or even be inevitable in them, because recurrent 

 mutation in certain genes appears to be more likely than in other 

 genes. But even if this view were maintained the inheritance would 

 be different in kind from the inheritance of ordinary Mendelian genes, 

 because such a view involves a secondary step, viz, the likelihood of 

 a mutation in a race containing the inherited gene in question. The 

 whole problem of the causes of mutation is at present so obscure that 

 a discussion of this possibility is purely theoretical. Added to this 

 is the uncertainty of how cancer is inherited in those races of mice 

 that appear to produce it with great frequency. Important as the 

 work along these lines unquestionably is, the subject is not yet ripe 

 for any positive statement. It may, nevertheless, be worth while to 

 keep in view the possibility suggested above, viz, that what is in- 

 herited in cancer may be a gene or complex of genes in which somatic 

 mutation is of sufficient frequency to give the appearance that a gene 

 for cancer is itself inheritable. 



IS THE FREEMARTIN A GYNANDROMORPH? 



It has been suggested that the pair of twin calves, one male, the other 

 a sterile female (the freemartin), together represent a sort of gynandro- 

 morph. This view is based on the assumption (which Lillie has since 

 disproven) that these twins arise from a single egg. Hart (Proceed- 

 ings Roy. Soc. Edinburg, XXX, p. 218) suggested that "the free- 

 martin with a potent bull twin is the result of a division of a male 

 zygote, so that the somatic determinants are equally divided and the 

 genital determinants unequally divided, the potent going to one twin, 

 the potent bull, the non-potent, genital determinant to the free- 

 martin." It is needless to point out that this vague statement can 

 not be brought into accord with embryological evidence, because 

 Lillie 's work shows that each individual of the twins arises from a 

 separate egg. In most cases the eggs arise from the two ovaries, and 

 each embryo lies in a different horn of the uterus. 



Lillie has shown that in those cases where twins are present, one of 



which is a freemartin, the two chorions and the two allantois have 



I fused at an early stage, and he has demonstrated that there is an 



