ONE-EGG TWINS IN FISHES 49 



When two individuals have arisen from two closely 

 approximated embryonic axes they may be crowded 

 together laterally so closely that the structures of the 

 inner sides, such as the pectoral and pelvic fins, may come 

 to be more or less fused and crowded out of place; but 

 this situation is only to be expected as the result of such 

 close quarters. This kind of fusion, however, is quite 

 different in principle frorn the sort of primary fusion 

 that Gemmill and others invoke in order to account for 

 double monsters. 



CAUSE OF SUBNORMAL DEVELOPMENT OF ONE TWIN 



When two embryos arise by plural gastrulation one 

 of the individuals is often subnormal throughout its 

 life. Is its subnormal condition due to an inhibition 

 exerted by the larger embryo, or is it the result of being 

 subordinate from the time of its origin ? My theory is 

 that in many cases the original axiate relations «have 

 persisted to some extent; that on this account any 

 individual arising from a secondary gastrulation area 

 was from the first subordinate and inhibited owing to 

 its origin from a region that still belonged, at least 

 partially, to the original axis of the single individual. 

 In extreme cases such a secondary axis is rather promptly 

 suppressed when the primary individual regains its 

 normal growth momentum. In less extreme cases the 

 secondary individual is able to maintain some degree 

 of independence, but is handicapped at first by its 

 slower start and secondarily by its contact with the larger, 

 more vigorous individual. 



In so far as the mode of origin of separate fish twins 

 is concerned, with the possible exception of those with 



