48 



THE PHYSIOLOGY OF TWINNING 



relative positions of these twins and their points of 

 attachment depend, I believe, upon how far apart on the 

 blastoderm the twin gastrulation areas occur. It may 

 be recalled that, in the duplicate twin starfish larvae, 

 the archentera sometimes occurred closely side by side, 

 and sometimes they occurred as far apart as possible. 



Figs. 27, 28. — Typical "separate" trout twins arising from double 

 symmetrical gastrulation. Fig. 2 7, a pair of identical twins, both normal. 

 Fig. 28, a pair of such twins, one of which is somewhat smaller and 

 defective in the head region. (After Stockard.) 



Doubtless the same is true for the fishes. If the two 

 individuals arise close together we would expect them 

 to be united by their inner sides; but if they arise on 

 opposite sides of the blastoderm, they should be facing 

 one another belly to belly. And there would be many 

 intermediate stages. Such unions never involve any 

 more than mere external fusions, axial elements such 

 as notochord, neural tube, and alimentary tract never 

 being united. 



