^V COS ^^(\ 



CHAPTER IX 



THE MODES AND CAUSES OF HUMAN TWINNING 

 INTRODUCTION 



We recognize two types of twinning in man — one- 

 egg twinning and two-egg twinning. In our present 

 discussion we shall limit ourselves to one-egg twinning; 

 for two-egg twinning is, strictly speaking, not true 

 twinning at all. 



It has now come to be very generally agreed that 

 separate one-egg twins (duplicate or identical twins) 

 belong to the same series and result from the same causes 

 as conjoined twins or double monsters. If this con- 

 clusion is valid, acceptable theories of the modes and 

 causes of twinning must be in conformity with conditions 

 in both types of twins. It is my belief that the clue to 

 the mode of human twinning must come from a study 

 of the various incomplete stages of twinning exhibited 

 by conjoined twins. 



THE MODE OF ORIGIN OF CONJOINED TWINS 



For a long time it has been tacitly assumed, and 

 rightly so, that conjoined twins are the products of some 

 kind of division of a single egg. The grounds for this 

 assumption are: (a) they are always of the same sex; 

 (6) they very frequently show situs inversus viscerum; 

 (c) they are usually cosmobia, i.e., they are joined in 

 symmetrical positions with regard to one another, and 

 homologous parts of the two systems are always united. 



