420 Duplicate Twins and Double Monsters 



forms of double malformation, the degree of duplicity, the character and 

 extent of the fusion, all result from the proximity and relative positions 

 of the neural axes of two more or less definite primitive traces developed 

 on the vitelline membrane of a single ovum." In this, by the supposi- 

 tion of a " double cicatricula/' he implies, to us at the present time, a 

 fission of the original germ, but, although he comes fairly up to this 

 point, he seems to have nowhere definitely expressed as much and such a 

 conception was clearly beyond him at that date. The " double cicatri- 

 cula " being present, he supposes the monster to be formed by a subse- 

 quent fusion of the " primitive traces." In true double monsters he 

 recognizes the following characters: (1) "homologous union" (i. e., a 

 symmetrical position of the two components) ; (2) unity of sex, and 

 (3) the transposition of viscera in one of the components, a condition 

 the necessity of which he somewhat doubts, although he gives several 

 instances of it. 



The step that Fisher did not take, that of accounting for the " double 

 cicatricula " in the first place by a fission of a single germ at an early 

 period, was proposed soon after the date of his paper, and probably orig- 

 inated with Dareste, although the idea seems to have been evolved inde- 

 pendently by Galton and others at about the same time. Dareste, how- 

 ever, was working upon the subject of double monsters and Galton upon 

 that of duplicate twins, and it seems to have been first applied to both 

 by Perls, 79, who starts in both cases with the fission of an originally 

 single germ, the two resulting parts of which normally become two sepa- 

 rate individuals, but which may, in rare cases, fuse secondarily into a 

 compound monster. Thus Blanc, 93, describes the origin of ischiopagi 

 as follows : " Les monstres ischiopages resultent, ainsi que I'a montre 

 M. Dareste, de la soudure de deux embryons, opposes par leur extremite 

 130sterieure et places dans le prolongement Fun de I'autre " (p. 238). 

 He then explains that if the two embr3ros fuse along a straight line the 

 legs and pelvic organs have room to develop " devies a droite et a gauche,'' 

 but that if they form an angle with one another the parts enclosed by 

 this angle suffer, owing to lack of space. 



Although not quoted directly from Dareste himself, it is altogether 

 probable that the above clearly represents his views, at least concerning 

 one type of diplopage. The same view is held by his associate, Bau- 

 douin, 91, who states that in the production of diplopagi the two primi- 

 tive streaks unite secondarily at a given angle with one another, and 

 that the variations depend upon (1) the precise epoch at which the 

 fission takes place, and (2) the size of the angle formed by the two axes. 



