154 THE BIOLOGY OF TWINS 



the heart apex turned to the right instead of to the left, 

 and the greater curve of the stomach turned to the 

 right. An extensive examination, however, shows 

 that no visceral reversals occur. In his book, Problems 

 of Genetics^ Bateson calls attention to the same situation 

 in human duplicate twins and says: ''If anyone could 

 show how it is that neither of a pair of twins has trans- 

 position of the viscera the whole mystery of division 

 would, I expect, be greatly illuminated." 



From what we know of the process of polyembryonic 

 fission in the armadillo, it would appear that the most 

 likely solution of this problem lies in the fact that 

 twinning is initiated and carried out in the ectoderm, 

 and the endoderm becomes involved only passively and 

 considerably later. What more natural, then, than 

 to look for evidences of twinning — mirror-image rever- 

 sals — only in ectodermal and closely associated structures 

 of the integument? In human duplicate twins it is 

 true also that reversals are confined to the friction 

 ridges, which are quite homologous in origin with the 

 armor characters of the armadillo. 



If Bateson should chance to read this suggestion 

 as to the reason why transposition of viscera does not 

 occur in twins, I doubt whether he would admit that 

 ''the whole mystery of division is hereby greatly illu- 

 minated." It is an interesting fact, however, that the 

 ectoderm, where the rate of metabolism is highest, 

 should take the lead and carry out the fission process, 

 thus imposing the results of fission on the tissues of 

 lower rate of metabolism, the endoderm and the meso- 

 derm. The ectoderm is dominant in early development 

 and produces the central nervous system through 



