6o THE PHYSIOLOGY OF TWINNING 



axis and the nature of the germ ring, but persists in the 

 idea that double monsters are the result of a fusion of 

 originally separate embryonic axes. His own statement 

 makes this clear: 



Approximation and union (of the twin centers of embryo- 

 formation) are due to the factors to which attention was directed 

 above, namely, the utilization during growth of the blastodermic 

 margins near the primitive streak, and the slowness of expansion 

 on the part of the blastoderm over the yolk in this same region 

 The twin adjacent axes are inevitably brought together posteriorly 

 through the disappearance of the interval between them. The 

 process may be called one of primary fusion, in contrast with a 

 process which often supervenes later, and which consists in the 

 secondary fusion of organs or structures already laid down. 



Primary fusion takes place earlier or later, i.e., in the head-, 

 body-, or tail-region, according to the interval which separated 

 the embryonic rudiments when they first appeared. In other 

 words, the degree of duplicity varies directly with the original 

 distance between the two centres of embryo-formation. When the 

 union is a purely lateral or an approximately lateral one, the pos- 

 terior united part finally becomes simply and perfectly bilateral. 

 This takes place through the gradual fusion and disappearance of 

 structures belonging to the left and right halves respectively of the 

 right and left component embryos. Thereafter the right and left 

 halves of the right and left embryos unite naturally to form a 

 normal bilateral body or tail. On the other hand, when the 

 twin bodies come together by ventral rather than by bilateral 

 union, the formation posteriorly of a perfect single body or 

 tail becomes impossible, since the necessary readjustments of 

 right and left structures in the twin embrj^os can no longer take 

 place. 



We see then that Gemmill is quite uncompromising 

 in his position that double monsters of all sorts are fusion 

 products. We have already pointed out that the fusion 

 theory is quite untenable and shall attempt to show later 



