MODES AND CAUSES OF HUMAN TWINNING 129 



essentially a fission theory similar to that proposed by 

 Assheton for his early sheep twins. Streeter considers 

 that the fission process takes place at the inner-cell-mass 

 stage. This mass or ''embryonic node" is believed to 

 undergo subdivision into two more or less equivalent 

 masses. If the two masses are equal in size 



then chances of developing in an orderly manner would be equal, 

 and this is presumably what happens in most instances of identical 

 twins. Where the secondary bud is merely a fragment of the 

 original mass we would expect that there would be some degree 

 of differentiation; but the process of development would soon be 

 arrested, and at term the stunted bud would be found as a small 

 epithelial cyst on the placenta near the attachment of the umbilical 

 cord. In case the twin-bud is only partially detached from the 

 primary node there would exist the basis for the various types 

 of double monsters and teratoma. 



It will be noted that Streeter uses the language of the 

 budding theory, probably influenced by myself and by 

 Patterson, but the process which he describes and figures 

 cannot rightly be called budding, especially if the 

 embryonic node divides into two equal masses, for in 

 this case we could hardly speak of one as the bud and 

 the other as the stock. As has already been said, the 

 theory in no way resembles the budding theory of 

 Patterson, but is definitely a fission theory. The chief 

 objections to Streeter's theory are that it fails to account 

 for the symmetry relations of duphcate twins and for 

 the fact that such twins frequently have a common 

 amnion. Moreover double monsters, which are beheved 

 to belong to the same series of twins, always have a 

 common amnion and have strikingly symmetrical and 

 intimate interrelations: conditions that could not be 

 accounted for unless the embryonic axis were either 



