ONE-EGG TWINS IN FISHES 



41 



nally single embryonic shield. It is not, as Gemmill 

 believes, the product of the fusion of two adjacent em- 

 bryonic shields. 



Twins originating from plural invaginations of the 

 margin of the germ ring must inevitably come to lie 

 parallel to each other with heads pointing in the same 



Figs. 23, 24. — Germ-ring stages of twin trout embryos. Fig. 23, an 

 embryo with two equivalent embryonic shields, destined to form separate 

 twins. Fig. 24, an embryo with two embryonic shields, one destined to 

 form a single separate individual and the other a tw^o-headed double 

 monster. (After Rauber.) 



direction. They must also have a common yolk sac 

 and yolk stalk. Such twins could therefore never 

 become fully separate. 



The question arises as to whether it would be possible 

 for fish twins to arise in such a way that they would 

 be separate when hatched. Klaussner (1890) cites one 

 case of fish twins which might possibly have become 

 separate after hatching. This pair of twins was found 

 lying side by side on one egg but with heads pointed in 

 opposite directions. Obviously such a condition could 

 not have arisen as product of the invagination of two 



