S6 



THE PHYSIOLOGY OF TWINNING 



in this I agree, that in this case ''two distinctly inde- 

 pendent chick embryos would have resulted at the end 

 of the incubation period." 



55BiEJt<;„.8^Tei%. 



Fig. 42. — A rare type of chick duplicity, probably the result of a 

 nearly complete longitudinal fission of an originally single embryonic 

 axis. There are no evidences of plural gastrulation but, on the contrary, 

 the posterior end of the axis is still single. (After Tannreuther.) 



4. Already under the second heading I have indicated 

 that there is some very good evidence that twin embryos 

 originating from separate embryonic axes and arising 

 directly opposite to each other on the same blastoderm, 

 may grow toward each other, meet head on, and fuse at 

 their anterior ends (Figs. 37 and 38). I see no other satis- 

 factory explanation of the crumpled condition of the two 

 anterior ends. Examples of such a condition have already 



