i6 THE BIOLOGY OF TWINS 



lutea, would be necessary to raise the phenomenon 

 of monozygotic twinning in man from the reahn of 

 probabihty to that of demonstrated fact. Even if 

 these data were obtained, we should still be far from 

 a real understanding of the mechanics of twinning 

 in man. 



THEORIES OF THE MODE OF TWINNING IN MAN 



Assuming that dupUcate twins in man are monozy- 

 gotic, how is twinning accompHshed ? In the quotation 

 of Schultze and from Wilder's comments we may glean- 

 a knowledge of practically all the various hypotheses 

 on this subject. Some of these are scarcely of suflScient 

 importance to deserve notice. Origin from a double- 

 yolked egg, for example, would scarcely be possible in 

 a mammal. Wilder rightly excludes the possible origin 

 of twins from binucleated eggs, because there would 

 have to be two sperms, and this would introduce a 

 degree of diversity that does not occur. Von KolHker's 

 suggestion that a single egg may occasionally produce 

 two embryonic areas which might or might not develop 

 separate amnions, depending on the degree of juxta- 

 position of the two areas, is well worth consideration. 

 It seems far more probable than Wilder's original theory 

 of the complete separation of the two blastomeres 

 resulting from the first cleavage division of a fertihzed 

 egg. Wilder has recently abandoned this extreme 

 "blastotomy" theory under the influence of the dis- 

 coveries of Newman and Patterson on the armadillo, 

 and is inclined to agree with these writers in their 

 suggestion that the origin of human dupUcate twins 

 probably resembles that of the armadillo quadruplets. 



