122 THE PHYSIOLOGY OF TWINNING 



Conjoined twins are united in a great , variety of 

 ways. By far the commonest condition is that in which 

 the anterior parts are separate and the posterior parts are 

 fused. There are, however, rare cases of twins, called 

 Janus monsters in which the heads are less completely 

 separate than are some of the more posterior parts of the 

 body. It is not fair to say that these individuals have 

 a single head and two complete bodies, for even the head 

 is double in that there are two faces. 



When twins are united they are usually connected 

 by ventral regions of the body though they may later 

 come to lie in such a way that they fail to show this 

 condition clearly. There is no certain case in which the 

 dorsal parts of the twins remain single while the ventral 

 parts are double. There is nearly always dorsal dupli- 

 city and ventral unity. Even in the cases of so-called 

 pygopagi, in which the twins seem to be joined back to 

 back, the conjoined organs are not the vertebral column 

 nor the central nervous system, but are nearly always 

 certain ventral structures such as the intestine or the 

 urethra. Rare cases occur in which otherwise com- 

 plete twins are lightly united in the head region, as for 

 example in the region of the forehead or the top of the 

 head. 



If, instead of attempting to classify and to interpret 

 these double monsters on the basis of the degree of more 

 or less mechanical union which they exhibit, we give 

 attention to the degree of duplicity of the various parts 

 of the body and of its organs, the problem becomes much 

 simpler. 



Are conjoined twins to be viewed as incompletely 

 fused separate individuals or as incompletely divided 



