126 THE PHYSIOLOGY OF TWINNING 



In 1904 Wilder in an important discussion of human 

 duplicate twins and double monsters proposes the 

 "blastotomy theory" of such duplicities. His idea, 

 which has already been discussed in The Biology of Twins 

 and need only be mentioned here, is that separate twins 

 result from the complete separation of the blastomeres 

 of the two-cell stage of the ovum, and that double 

 monsters result from incomplete separation of these 

 blastomeres. The degree and position of the union 

 between these twins are attributed to variations in the 

 points of contact of the two cells. If they remain 

 attached by apical ends, we would have Janus monsters; 

 if by the basal ends we would have pygopagi; if by 

 ventral sides, thoracopagi. Since the discovery of the 

 mode of twinning in the armadillos Wilder himself has 

 abandoned his view in favor of the "budding theory." 

 If, however, the budding theory turns out to be inade- 

 quate for the armadillos, there is even less reason for 

 its adoption in the case of human one-egg twins, and 

 especially is it inapplicable to that of conjoined twins. 



streeter's theory of the origin of 

 human twins 



In view of the fact that no really early cases of one- 

 egg twinning are known for man, the theory has pre- 

 vailed that the process must be closely similar to that 

 of the armadillo. The facts that in both man and the 

 armadillo the uterus is simplex, that there is a similar 

 ectodermic mass and subsequently a similar ectodermic 

 vesicle involving a similar method of amnion formation, 

 have made it seem highly probable that twinning in man 

 is equivalent to the first step in twinning in the armadillo 



