MODES AND CAUSES OF HUMAN TWINNING 123 



single individuals? The decision between these alter- 

 natives is crucial for our theory. 



Granted that they are derived from a single egg, do 

 the two parts of such twins come from two independently 

 arising embryonic axes which subsequently come to fuse 

 together in certain regions and remain separate in others; 

 or do they arise as the result of a more or less complete 

 separation of the bilateral halves of a single embryonic 

 axis? The first alternative, which involves the idea of 

 fusion of separate embryos, meets with an almost insuper- 

 able obstacle on account of ''the complete bilateral 

 symmetry of the two components in true double monsters 

 (diplopagi), since there is no force to oversee and adjust 

 the two components in the exact relationship necessary 

 for this result" (Wilder, 1904). There may be, how- 

 ever, a minor degree of purely external or mechanical 

 fusion due to previously separated parts remaining too 

 closely approximated. Such embryos may be pushed 

 or crowded together, for they lie within a single amnion 

 and have no means of avoiding contacts. For enlighten- 

 ment as to the mode of origin of human double monsters 

 we may profitably turn to the much better understood 

 conditions in the birds; for we have reason to believe 

 that mammalian conditions are quite similar in most 

 respects to those of reptiles and birds. There were, it 

 will be recalled, two types of avian double monsters in 

 which there was more or less unity of the head regions 

 and duplicity of the trunk region. The first of these 

 types was explained on the basis of the existence of two 

 more or less separate bilateral growing regions, the head 

 process and the primitive streak. Either region may 

 undergo complete or partial twinning without the co- 



