MODES AND CAUSES OF HUMAN TWINNING 131 



12 mm. Each had its individual umbilical cord; these were 

 attached to the chorionic wall, a quadrant's distance apart. 

 Adherent to the amnion of one embryo was a yolk sac of normal 

 size The other embryo has no yolk sac. 



Arey enters into a discussion as to the bearings of the 

 lack of yolk sac in the latter twin, which seems to be of 

 little value for our theory. Our chief concern has to do 

 with the mode of origin of these two cases of human 

 twins. The first case is almost certainly a case of double 

 gastrulation of a distinctly symmetrical sort like those 

 of the starfish shown in Figures 4-6 or like certain chick 

 twins such as that in Figure 36. The mode of origin 

 of the other twin embryo is uncertain. Since the two 

 embryos are not attached to the same yolk sac they have 

 probably originated from an early total fission of the 

 blastoderm or embryonic node much like the hypothetical 

 case of Streeter except that the fission must have resulted 

 in two practically equal primordia both of which were 

 able to form a placenta. 



MODES OF HUMAN ONE-EGG TWINNING 



Although the evidence is still somewhat meager we 

 are now in a position to state with some confidence that 

 the same three modes of one-egg twinning occur in the 

 case of man as have been described for previous inverte- 

 brate and vertebrate types: (a) twins produced by 

 fission of the blastoderm, as illustrated by Streeter's 

 case and the second case of Arey; (b) twins produced 

 by double gastrulation, as in Arey's first case; (c) 

 double monsters, and possibly some entirely separate 

 twins, produced by partial or complete fission of the 

 bilateral halves of a single embryonic axis. 



