84 



THE PHYSIOLOGY OF TWINNING 



fusion idea are the same in this case as in other cases pre- 

 viously considered. The condition is, in my opinion, 

 unquestionably a result of a process of fission of the 

 posterior growing region of the embryo. It must be 



Fig. 41. — A typical two-tailed chick double monster (katadidjonus) 

 due to the partial fission of the posterior end of a single embryonic axis. 

 (After Tannreuther.) 



supposed that at the time when the agencies responsible 

 for twinning were applied, the head parts of the sym- 

 metry axis had been definitely established and that there 

 was a secondary point of great activity and high suscep- 

 tibility at the level of the dorsal lip of the blastopore which 

 is probably at the anterior end of the primitive streak. 



In the chick there appear to be really two distinct 

 regions of differentiation and two gradients: one the 

 head process and the other the primitive streak. The 



