Harris Hawthorne Wilder 397 



Eelation of Duplicate Twins to Double Monsters. — The fact 

 that in these experiments with invertebrates an incomplete separation of 

 the components will produce various types of double monsters suggests 

 that certain instances of these latter among vertebrates such as have been 

 especially recorded in the case of man and other mammals, may be due 

 to a similar origin. The opposite principle, also, that of the fusion of 

 what were originally either separate eggs or separate blastoderms, seems 

 ■also to obtain in some instances, as would be very apt to be the case 

 where two blastoderms are enclosed by a single amnion (Schiiltze's 

 Case IV), " ein Fall . . . der den nachsten Uebergang zu den Doppel- 

 missbildungen darstellt," but in this case the two resulting compounds 

 would not be symmetricaly joined nor of equal development. 



Aside from these and similar speculations, nothing is definitely known 

 concerning the real origin of either equal or unequal composite monsters, 

 although these phenomena have been a favorite subject for speculation in 

 all ages, and have given rise to numberless theories. Of these the most 

 plausible seem to me those based upon the experiments with the eggs of 

 invertebrates and upon the intra-uterine relations just considered, but, 

 although we have these phenomena on the one hand to serve as causes, 

 and the various types of composite monsters as results, the connection 

 between the two is still mainly a matter of conjecture, and we are far 

 from being able to explain definitely the relations between the various 

 causes and the equally varied results. It is thus merely as a working 

 hypothesis, upon which to base the facts presented later on in the paper, 

 that I shall attempt here a discussion of the relationships of twins of both 

 sorts to composite monsters, the relationships of the various types of 

 these monsters to one another, and the probable causes which lead to the 

 production of each type. 



To present the material for this discussion before the reader, the fol- 

 lowing list of recorded instances has been compiled, whenever possible 

 from the descriptions of actual observers; much also has been obtained 

 from the various compilations referred to at the beginning of the bibliog- 

 raphy. To all these sources I wish to acknowledge my indebtedness. 



The classification adopted for this list is purely a geometrical one, and 

 differs but little from that of other authors, the main attempt being to 

 arrange the material in a convenient form for later reference. Later on, 

 in discussing the origin of these monsters, they will be arranged in accord- 

 ance with their probable physiological relations, in an attempt to show the 

 causes of these phenomena. These relations are expressed also in the 

 general diagrams (Plate A.). 

 29 



