466 HAROLD HEATH 



develops laterally the two well known enteroceles, so that the 

 median section between the pouches may possibly be considered 

 an incipient unpaired vesicle as certain authors have suggested. 

 On the other hand it is equally possible that some of the mesen- 

 chyme cells migrating from the distal end of the archenteron 

 represent the anterior vesicle though they may never actually 

 fuse. In Pateria mineata, for example, not over 50 per cent 

 of the embryos form a complete vesicle; in a few cases, rela- 

 tively, it is imperfect, and it appears probable that in the others 

 it is represented by isolated mesenchyme cells which arise in 

 the proper position but never unite. Furthermore, it is a signifi- 

 cant fact that in all of. these cases, even where the anterior 

 vesicle is perfectly formed, the enterocele pouches are separated 

 by a thin section of archenteric wall which therefore can scarcely 

 be regarded as representing the anterior vesicle. 



Gemmill's discovery of a pair of posterior enterocele pouches in 

 the starfish is of the highest importance. They have the same 

 origin as the trunk coelom in Balanoglossus, and as Gemmill 

 states "We have here, I think, rudiments of a paired posterior 

 enterocoelic outgrowth, which in the common ancestor of 

 Balanoglossus and the Echinoderms gave rise to the coelom of 

 the body or trunk." In some of the species studied by Gemmill 

 "this rudiment takes no part, or only a small part, in the forma- 

 tion of the wall of the posterior coeloms, but it still retains the 

 function of producing mesenchyme." In Pateria mineata, on 

 the other hand, it is a relatively large vesicle, present on the 

 left side only, and in the future an attempt will be made to de- 

 termine its fate. It is possible that, in this species and in the 

 others where it forms mesenchyme, this posterior outgrowth does 

 play a more or less important part in the history of the left entero- 

 cele from which the hydrocele arises, and that some of the diffi- 

 culties in homologizing this last named cavity are due to its 

 compound character. 



To sum up : I am strongly inclined to look upon the anterior 

 vesicle in Pateria mineata as the homologue of the proboscis 

 coelom of Balanoglossus, while the posterior outgrowth corre- 

 sponds to the trunk coelom, and the intermediate pair of vesicles 



