DOUBLE MONSTERS IN FISHES 65 



which gives rise to a smaller, somewhat inhibited embryo 

 that may subsequently come to be hardly more than a 

 parasite on the body of the primary individual. This 

 would be my interpretation of most of the cases of 

 parasite and autosite described by Stockard and others 

 (Figs. 25 and 26). Windle has cited a case in which the 

 parasite has been reduced to a mere bump on the side 

 of the autosite. If we were using the terminology of 

 budding it would be fair to consider such a parasite as 

 the product of a ''secondary bud," because the primary 

 ''bud" has retained its identity. When, however, the 

 twin axes are both new and equivalent, neither would 

 be a secondary "bud." Though, as we have seen, the 

 budding conception seems far fetched and valueless, it is 

 relatively unobjectionable when restricted to separate 

 twins. It is when this concept is carried over to the field 

 of conjoined twins that we find it utterly untenable. 



THE LATERAL BUDDING THEORY OF THE 

 ORIGIN OF CONJOINED TWINS 



Stockard notices, as had many others before him, 

 that the two components of a pair of conjoined twins 

 are frequently of unequal size. As a rule the larger 

 component is practically normal, while the smaller 

 exhibits various evidences of having suffered inhibition. 

 Such smaller components frequently show the same 

 types of abnormality (cyclopia and similar defects) as are 

 seen in single embryos that have been exposed at an early 

 period to growth-retarding agents. Stockard interprets 

 this situation somewhat as follows. The normal com- 

 ponent is thought of as arising from a primary embryonic 

 shield which would have formed a single embryo but 



