74 THE PHYSIOLOGY OF TWINNING 



period when it is readily arrested. A few anomalies are 

 interpreted by Dareste as the result of developmental 

 excess, a sort of supernormal development. Some types 

 of monsters are interpreted as the result of adhesions and 

 of unions of similar parts. 



The part of Dareste's treatise which especially inter- 

 ests us at present is that in which he deals with the origin 

 of double monsters and twins. He reviews the history 

 of theories of the origin of double monsters, theories that 

 date back as far as the beginning of the eighteenth century. 



The anatomist Duverney in 1706 published an 

 account of the organization of an ischiopagus human 

 monster and expressed the conviction that it could not 

 have arisen through the partial fusion of two embryos 

 but must have pre-existed as a double monster in the 

 egg. He was imbued with the prevalent doctrine of 

 preformation and therefore found the idea of a completely 

 preformed double monster more acceptable than one 

 involving epigenetic changes. 



In 1724 Lemery, on the basis of another dissection 

 of a two-headed, single-bodied human double monster, 

 sought to prove that such monstrosities could not have 

 been preformed but must have resulted from two separate 

 embryos derived from two eggs. 



Winslow came to the support of Duverney's position 

 and opposed the fusion idea on the grounds that the 

 double monsters were symmetrically united and that one 

 of the components showed situs inversus viscerum, a 

 condition impossible to account for on the basis of the 

 fusion theory. 



Wolff (1772) combated the doctrine of preformation 

 and revived the epigenesis doctrine of Harvey. He 



