POLYEMBRYONIC DEVELOPMENT IN TATUSIA 641 
now extinct is, of course, not known; but it can be stated that 
some of the living species, other than T. novemcincta and T. 
hybrida, are known to give birth sometimes to a single young, 
and at other times to two individuals. 
8. Polyembryony and duplicate twins and double monsters 
Another interesting problem with which polyembryony is con- 
cerned is that of the origin of duplicate twins in the human species. 
There is a vast literature bearing on duplicate twins and various 
types of double monsters, and several different theories have been 
advanced to account for their production. The reader is referred 
to Wilder’s (’04) extensive paper in which the more important 
references are cited, and in which the leading theories are dis- 
cussed. 
It is well at first to distinguish between the different kinds of 
twins, ‘duplicate twins’ and ‘fraternal twins’, understanding by 
the former those cases in which the two members of a pair are 
supposed to have come from a single egg, and by the latter those 
supposed to be the product of two distinct eggs. This distinction 
is by no means an artificial one, but is based upon a considerable 
amount of data. It is supported not only by the general physi- 
cal appearance of the members of a pair, but also by the intra- 
uterine relationships of the two members. In fraternal twins 
the two individuals do not resemble each other any more closely 
than do the several individuals of a litter belonging to a normally 
multiparous animal, and the intra-uterine relationship of two 
chorions indicate that their origin is from two distinct eggs. In 
duplicate twins the individuals are enclosed within a single cho- 
rion, and their close resemblance to each other is often so striking 
that it has gained for them the name of ‘identical twins.’ Fur- 
thermore, duplicate twins are invariably of the same sex, while 
fraternal twins may be of the same or of different sex. 
Wilder points out that this fundamental principle upon which 
the distinction between duplicate and fraternal twins is based, 
also holds in multiple births involving more than two individuals, 
and that it can be extended to include cases of duplicate twins 
and similar combinations in other forms. 
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