1 8 THE BIOLOGY OF TWINS 



Since it seems entirely probable that human duplicate twins are 

 separated at a much earlier period than are armadillo quad- 

 ruplets, it may not be unreasonable to look for this separation at 

 some period of cleavage. Or there may be a division of the inner- 

 cell mass into the primordia of the two embryos. The problem 

 of the exact origin of duplicate human twins is, however, likely 

 to remain unsolved for a long time to come. 



CONJOINED TWINS AND DOUBLE MONSTERS 



Double individuals have for a long time attracted 

 the attention of obstetricians and embryologists, and 

 there is an extensive literature on the subject, with 

 little value, however, for a book of this sort. Certain 

 of the lower-grade conjoined twins, the Siamese twins, 

 for instance, have stimulated human curiosity to such 

 an extent that they have been exploited as freaks in 

 circus side shows and museums the world over. The 

 late P. T. Barnum owed a considerable measure of his 

 early success to his discovery and exhibition of the 

 Siamese brethren. Every degree of junction between 

 twins has been noted, ranging from mere fusion of 

 parts of the skin that can be and have been cut apart 

 without injury to extreme unions involving the head 

 and entire trunk. An abbreviated classified list of 

 types of composite monsters, based on Wilder's ex- 

 tensive data, will serve to show the range of forms 

 included. 



There are distinguished two main types: one ''in 

 which the components or component parts are equal 

 to and the symmetrical equivalents of one another: 

 Diplopagi^'' ; the other, ''unequal and asymmetrical 

 monsters, one component of which is smaller than and 

 dependent upon the other: Autosite and Parasite. 



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