DOUBLE MONSTERS IN FISHES 6i 



that the dichotomy or fission theory much more nearly 

 satisfies the conditions observed. 



Undoubtedly a certain amount of secondary fusion 

 of external adjacent structures does occur, as for example 

 in such cases as that described by Windle in which the 

 tails, crossed the one over the other, are united for a 

 short distance by the fusion of the ventral side of one 

 with the dorsal side of the other. Apart from such 

 obvious cases and such cases as those referred to on page 

 48, where adjacent identical twins come to fuse side to 

 side by the body wall, we may safely abandon the fusion 

 theory of the origin of true double monsters. 



stockard's theory of the mode of origin of 

 twins and double monsters in fishes 



In his recent monograph Stockard (192 1) undertakes 

 to explain the morphology and physiology of twinning 

 in the fishes, the basis of his theory being certain experi- 

 ments in artificial twin production in Fundulus. The 

 starting-point of his work was evidently my own theory 

 of the causes of twinning in the armadillo. He also 

 adopts Gemmill's theory of the origin of double monsters, 

 Patterson's ''budding theory" of twin origin, and Child's 

 theory of dominance and subordination. 



In so far as Stockard's theory depends upon Gemmill's 

 idea of origin of double monsters by fusion and upon 

 Patterson's budding theory of the origin of armadillo 

 quadruplets, his arguments and conclusions are unsound. 

 Quite apart from the fact that his theory has been 

 erected upon these two insecure foundations, Stockard's 

 own extensions and applications of these theories require 

 additional scrutiny. 



