TWINNING IN BIRDS 



87 



been described on pages 79-81. In these embryos the 

 brains are much folded and wrinkled as though each had 

 interfered with the other's growth. 



5. Many authors have described advanced twin 

 embryos in which the autosite and parasite condition is 

 obvious. In my opinion this condition may arise from 

 either of the two categories numbered (2) and (3), but 

 is much more likely to arise from (2). An unusually 

 interesting case (Fig. 43) of autosite and parasite in the 

 making is one in my own possession. This curious pair 



Fig. 43. — A very unusual type of chick twin embryo, doubtless a 

 case that would lead to the autosite-parasite condition. The two 

 embryos probably arose from two separate points of gastrulation, one 

 of which was primary and the other secondary. The smaller, secondary, 

 axis has evidently been partially inhibited by the larger, primary, axis, 

 and was destined to be a mere parasite on the body of the latter. 

 (Original.) 



