VARIOUS KINDS OF HUMAN TWINS 17 



The theory that duplicate human twins arise by some 

 sort of early fission process, probably initiated in the 

 ectoderm, is rendered probable by the facts observed 

 in the earliest known human embryos, notably those 

 of Peters, of Frassi, and of Bryce and Teacher. It 

 seems certain that the amnion in the human embryo 

 is not a folding of the somatopleure, but arises as a 

 hollow in a ball of ectoderm, as in the armadillo. This 

 type of amnion formation would furnish the requisite 

 mechanism for the production of ectodermic outgrowths 

 from which, as in the armadillo Dasypus, the primordia 

 of twins could take their origin. 



Though abandoning the crude idea of "blastotomy" 

 origin of dupUcate twins, Wilder still clings to the idea 

 that the hereditary differentiation between the twins 

 is to be traced back to events taking place during the 

 first cleavage. In this he agrees with my own position 

 taken in connection with armadillo quadruplets. 



My opinion regarding the mode of origin of human 

 duplicate twins was stated in the concluding paragraph 

 of my latest paper on "Heredity and Sjntnmetry in 

 Armadillo Quadruplets":' 



I am inclined to believe that duplicate human twins become 

 physiologically isolated at a considerably earlier period than do 

 armadillo quadruplets, and niy reason for this belief is founded 

 on' the fact that there is so little mirror-imaging in the former and 

 so much in the latter. It appears to be a good general rule that 

 the earlier the separation the more complete is the reorganiza- 

 tion of symmetry relations in the separate individuals and the 

 less residuum of the original common symmetry. Double mon- 

 sters doubtless begin to separate comparatively late in ontogeny 

 and hence (sometimes) show very pronounced mirror-imaging. 



' H. H. Newman, Biological Btdktin, XXX, No. 2 (1916). 



