MODES AND CAUSES OF HUMAN TWINNING 125 



THEORIES OF THE MODE OF ORIGIN OF 

 HUMAN DOUBLE MONSTERS 



In reviewing Dareste's work on the origin of double 

 monsters in the chick we had occasion to present a brief 

 history of European opinion as to mode of origin of 

 human double monsters (see pp. 74-76). These views 

 need not be restated here. Suffice it to say that the 

 weight of opinion was in favor of an origin by fusion of 

 separate embryos. In America, however, we find an 

 early expression of the fission theory. In 1866 G. H. 

 Fisher postulated a theory that human double mon- 

 strosity is due to an early total fission of the embryo, 

 followed by a subsequent fusion of the two parts. He 

 says that double monsters ''are invariably the product 

 of a single oyum, with a single vitellus and vitelline 

 membrane, upon which a double cicatricula, or two 

 primitive traces are developed. The several forms of 

 double malformation, the degree of duplicity, the char- 

 acter and extent of the fusion, all result from the prox- 

 imity and relative positions of the neural axes of the two 

 more or less definite primitive traces developed on the 

 vitelline membrane of a single ovum." This idea, the 

 reader will note, implies that all united parts of double 

 monsters are fusion products, a view quite inadmissible 

 in view of the various facts already stated and that are 

 soon to be discussed. The fission idea is also far from 

 clear. We are not told when or how the two primitive 

 traces originate. Fisher uses the word "fission" loosely 

 to mean some sort of dividing process giving rise to two 

 embryonic areas on one egg. He has no conception that 

 even remotely resembles that involved in our theory of 

 fission which has been several times stated. 



