VARIOUS KINDS OF HUMAN TWINS 15 



conclusion which is actually warranted by the facts is 

 that there are two kinds of human twins, fraternal (dizy- 

 gotic) and duplicate (monozygotic). There is such a 

 diversity of conditions, however, that it is impossible 

 in some cases to decide whether a given set belongs to 

 one or the other category. This situation is in marked 

 contrast with that seen in the armadillos (chap, ii), 

 where the intra-uterine relations are practically uniform 

 in all pregnancies, indicating that the peculiar mechan- 

 ism which brings about twinning is rigidly standardized. 

 In human twins, however, twinning is by no means 

 a single, fixed process, but is highly variable, evidently 

 beginning earlier and being more complete in some 

 cases than in others. Wilder is probably correct in 

 considering that the various symmetrical types of double 

 monsters belong to the same series as separate duplicate 

 twins. I would suggest that they are merely more or 

 less incompletely separated monozygotic twins, in which 

 the twinning process begins later than in the separate 

 twins and then fails to be fully carried out. 



Further evidence that duplicate twins are monozy- 

 gotic is derived from a study of identity between twins 

 and certain cases of symmetry reversal seen between 

 them. These matters will be taken up in a subsequent 

 chapter. 



There is, as we have seen, some embryological 

 evidence derived from the fetal membranes of certain 

 twins that they are monozygotic. There are also 

 certain twins strikingly identical. Unfortunately, how- 

 ever, there is no case in which both kinds of data have 

 been secured for the same sets of twins. This is the 

 information which, together with the data on the corpora 



