392 ARTHUR T. MASTEKMAN. 



There can be no question that these cells are segmented off 

 from tlie hypoblast in this region, and in fact could even be 

 regarded as being still integral parts of the hypoblast. I 

 cannot say for certain whether these two masses of cells have 

 archenteric grooves opposite them or not. It is possible 

 that this may be the case, and even that they are formed by 

 two archenteric diverticula which have been disguised by 

 shrinkage and imperfect preservation. The development o£ 

 these posterior mesoblastic masses is later than that of the 

 collar, for in fig, 32, a section a little ventral to fig. 31, the 

 collar cavities have expanded to form a median ventral 

 cavity. The posterior masses still lie in a dorso-lateral 

 position above the gut, and very little further developed in 

 the latest stages ; there can be little question that they give 

 rise to the coelom of the trunk, as found in late Actino- 

 trocha. 



Fio-s. 34 — 39 are a selected series of transverse sections 

 through the stage depicted in fig. 15. Figs. 34 and 35 show 

 the pre-oral coelom {pc.) containing its cavity, and giving 

 off its two lateral horns. These horns proceed backwards 

 on either side of the mouth, and their terminal portions are 

 cut across in fig. 37 level with the mouth. Further back, in 

 fig. 38, the collar mesoblast has fused ventrally, and formed 

 a spacious collar cavity {msc.) extending up laterally towards 

 the dorsal surface. 



In fig. 39 the posterior tip of the collar coelom is cut ven- 

 trally, and the paired masses of trunk coelom (mtc.) are seen 

 lying dorso-laterally to the gut. They contain no cavity, 

 and are small. 



Later Development. 



Returning to fig. 15, we find that further external differ- 

 entiation results in (fig. 16) the bifurcation of the posterior 

 end to form the rudiments of the two first tentacles and a 

 further growth of the pre-oral hood. Round the edge of this 



