ONE-EGG TWINS IN FISHES 



47 



larva was one in which the original polarity of the blastula 

 had been largely obliterated and in which two new twin 

 gastrulation areas had arisen, each of which is a secondary 

 area and each quite definitely a mirror-image duplicate 

 of the other. Neither one is primary to the other and 

 neither one tends to inhibit the development of the other. 



Figs. 25, 26. — Typical examples of trout twins of the "autosite- 

 parasite" variety. In both cases the autosite is nearly normal and the 

 parasite decidedly subnormal. (After Stockard.) 



They grow at equal rates and form identical twin axes. 

 The condition may be said to be due to the physiological 

 isolation of the two halves of the original blastoderm. 



Now just such a process as this takes place, I believe, 

 in the fish blastoderm and accounts for the numerous 

 cases of duplicate twins attached only by the yolk sac 

 or by parts of the lateral body wall. The two individuals 

 are each complete and normal and of approximately 

 equal size. Examples of such duplicate twins are seen 

 in Figures 27 and 28. The differences with regard to the 



