138 WHITMAN. [Vol. I. 



(each represented by a nucleated mass of protoplasm without 

 cell-boundary) ; the third by an exceedingly thin layer oi flat- 

 tened epithcliiim ; and the fourth by a columnar cpitJieliinn. 



3. The development of the mesenteron begins at the anterior 

 end, and progresses towards the posterior end, but more rapidly 

 along the ventral than the dorsal side. 



4. The phases of development are essentially the same as in 

 Rhynchelmis, the chief difference being that, in the latter, the 

 three primary entoblasts a, b, c, split up into secondary ento- 

 blasts before the entoplastic phase appears. 



5. The history of the mesenteron in Nephelis is very imper- 

 fectly known, but there is nothing in the observations thus far 

 published which appears to be irreconcilable with the results 

 obtained in Clepsine. The development in Clepsine is more 

 complicated, owing to the larger amount of food-yolk. It is 

 doubtful whether the entoplastic phase is represented in 

 Nephelis. 



6. It is possible that the residual mesoblasts (the remnants 

 left after the completion of the germ-bands) contribute to the 

 formation of the mesenteron. Such a termination of their his- 

 tory has not been ascertained, but is suggested by the fate of the 

 posterior macromere in Rhynchelmis. 



7. The proboscis — the homologue of the muscular pharynx 

 of the Gnathobdellidae — is lined with cells of entodermal 

 origin. The rest of the proboscis, together with the proboscid- 

 ial or pharyngeal pocket, is derived from the stomodaeal 

 thickening of the ectoderm. 



8. A knowledge of the history of the teloblasts clears up 

 many obscurities in regard to the origin and relations of the 

 germ-layers, particularly the entoderm and mesoderm. If the 

 precise origin of the teloblasts be known, that of the entoderm 

 may be inferred, and vice versa. 



