418 Duplicate Twins and Double Monsters 



FORMS WERE StAGES IN A CONTINUOUS DEVELOPMENT, AND ONE MUST 



BE ON HIS Guard not to look upon them as such. Instead of this 

 they form the varying steps in a process in which each case is fixed, 

 probably from the beginning, and represents but one step, and the pro- 

 cess is continuous only in the sense that the various possible cases form 

 a series of almost imperceptible gradation between two limits. 



The hypothesis advanced here necessarily presupposes some form of 

 preformation in the egg-cell and in the early blastomeres, but no more 

 than must be granted by any one who studies the various correspondences 

 found in separate duplicate twins, for example, the palm and sole con- 

 figuration. The main factor, however, in the formation of either a 

 diplopage or a duplicate twin is, however, the changed relationship of 

 the cells, a cell or a part of a cell developing differently when isolated 

 than when in contact ivith another. A sea-urchin hlastomere in the 

 2-celled stage, ivhen in its normal relation to its fellow, develops hut one 

 side of a bilateral larva, hut when this contact is severed, it develops 

 hoth. Similarly, if the relation is lost in part and retained in part, 

 a douhle monMer is the result from the same cause. 



It is manifestly impossible thus to experiment with human eggs or 

 with those of other placental mammals, but the results of observation 

 indicate that what is true in the case of lower forms, obtains also here. 

 In mammals the development of the extra-embryonal parts appears at 

 first to greatly complicate the problem, and they certainly do introduce 

 an element which cannot be wholly explained as yet in this connection, 

 but the facts in the case are so numerous, and the relationship of the 

 various forms of diplopagi are so simple and so completely analogous 

 to the results of experiment among lower forms that the close similarity 

 between the two cannot well be doubted. 



There yet remains to be accounted for those cases of compound mon- 

 sters in which the two components are very unequal, sustaining the rela- 

 tionship of parasite and autosite. These are plainly the result of a 

 secondary fusion, probably at an early age, of two embryos developing in 

 close proximity to one another, perhaps in the relation described in 

 Schultze's Case lY. The components, in order to be in this relation to 

 one another, must be originally duplicate twins, and as a matter of fact, 

 in so far as the parasite possesses sex, it is always that of the autosite. 

 It may be possible, of course, that under certain circumstances fraternal 

 twins may fuse with one another, but the enclosure in separate chorions, 

 which seems to be the rule, would normally prevent it. 



As stated above, no tests seem ever to have been made in such cases 

 relative to their original physical identity, although in many cases, such 



