14 THE BIOLOGY OF TWINS 



''Other Recent Theories Concerning the Genesis of Composite 

 Monsters. " — Wilder.) 



Case IV. — Similar to (III), but with both embryos inclosed 

 in a single amnion. This is a very rare case, explicable only by 

 postulating a single blastodermic vesicle upon which the two 

 embryonal areas are nearly or entirely in contact with one another, 

 a case which has been described by several authors as occurring 

 in the hen's egg. In such a case there would be an almost irre- 

 sistible tendency toward the fusion of the two embryos along 

 the line of mutual contact, thus producing some form of composite 

 monster. [Schultze says Doppelmissbildungen, but I use the 

 word double in a more restricted sense, as explained below.] 



(As Case II is seen to be a variation of Case I with the two 

 enibryos nearer together, so Case IV is seen to be a similar varia- 

 tion of Case III, with a similar result, i.e., the more complete 

 fusion of the parts, although here, owing to the direct connection 

 of the two embryos, the fusion is liable to extend also to these 

 and produce abnormal results. There are thus primarily not 

 four, but two cases, corresponding to the two types of twins, 

 fraternal and duplicate. The close connection of IV and III 

 suggests what may have already occurred to the reader: that 

 many cases of compound monsters come under the same category 

 as separate duplicates. This is quite probable, but such forms, 

 arising from a secondary fusion, would be more asymmetrical and 

 more or less unequal, and would come under the class of autosite 

 and parasite rather than that of symmetrical, or genuine double 

 monsters. — Wilder.) 



This rather extensive quotation from Wilder's mon- 

 ograph on Duplicate Twins and Double Monsters is 

 given largely to show the inadequacy of the embryo- 

 logical data upon which conclusions as to the mode of 

 origin of twins are based. In addition, one will readily 

 gain the impression that there is a very wide diver- 

 sity of interpretations of the facts, based largely up- 

 on assumed analogies with conditions found in other 

 vertebrates and even invertebrates. About the only 



