VARIOUS KINDS OF HUMAN TWINS 23 



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

 developed on the vitelline membrane of a single ovum. 



Considering how little was really known of embry- 

 ology at the time, this idea of Fischer shows surprising 

 insight. I am inchned to beheve that this explanation 

 comes very close to the true one. It will be remembered 

 that in the account of intra-uterine relations of dupHcate 

 twins a condition was described in which the twins were 

 not only monochorial but mon-amniotic (contained 

 within a single amnion) . This appears to me to present 

 many possibihties for fusions. If we suppose that 

 twins arise by some process of fission, in general hke 

 that in the armadillo, it is likely that in some cases the 

 outgrowths arise so close together that only a single 

 amnion is formed, and in such cases the visceral parts 

 in particular would be hkely to fuse or remain unsep- 

 arated, as in the more pronounced types of diplopagi. 

 Even in those cases that succeed in maintaining a dis- 

 junction until the body parts are completely separated 

 it would be possible, or even highly probable, that, 

 through the crowding incident upon growth within one 

 amnion, surface fusions more or less extensive would 

 occur. Wilder offers as an objection to this and other 

 theories involving the idea of secondary fusion of 

 originally separate primordia, that it is incompatible 

 with the fact that the two components of a double 

 monster are strictly bilaterally symmetrical and that 

 "there is no force to oversee and adjust the two com- 

 ponents in the exact relationship necessary for the 

 result." This objection loses force, I beheve, if the two 

 components are viewed as outgrowths lying in a bilateral 

 position on the germ and held in that position by their 



