OBSERVATIONS OX FKESH- WATER i'OLYZOA. 127 



author the cystogenous sphere, which consists solely of cells 

 of ectodermiil origin, is the first to form and to this is added 

 the Bildungsniasse by proliferation of (mesodermal) cells of the funi- 

 cular wall. Provided that in either case the two [x^rtions are respec- 

 tively ectodermal and mesodermal products, it would be of but 

 secondary importance whether they are distinct from the beginning 

 or become outwardly indistinguishable for a time. More study of 

 this point is exceedingly desirable. 



Further history of tlie development corresponds in the main 

 with what is already known. The cells of the two portions are con- 

 si'antly increasing in number and the entire mass in size. Meanwhile, 

 the cystogenous cells attain the character of columnar epithelium ; the 

 whole cystogenous sphere flattens, and s(-on takes the form of a shallow 

 watch-glass, the internal cavity disappearing (hg. 40, PI. XIX. ajst. c.) 

 We may speak of it as the cystogenous cu|). The C(jncavity of the 

 cu[) grows deeper, always closely clasping the mass of the remaining 

 cells, i.e., the " Bildungsmasse." The cells of the latter begin to 

 présenta granular a[)pearance by the deposition of refractile «pherules 

 in the protoplasm, comparable in nature to the deutoplasm of eggs 

 or of 3olk-cells in Plathelminthes. Braem could not convince him- 

 self of the truth of Nitsche's and Verworn's opinion that the granules 

 are direct products of the nuclei ; nor cou.ld I lind any sup[)ort to this 

 view. x\1jout this stage, the cells in cjuestion assume a spindle-;?hape, 

 the axis standing vertical to the cavity of the cystogenous cu)) (tig. 

 o'J, PI. XIX, gr. m.). This state was also noticed by Braem in Cris- 

 tatella. H<3wever, as the granulation advances, they become rounder 

 again, until each cell is represented by a globular mass oi' granules 

 with a nucleus at the centre (fig. -il, PI. XIX, gr. in.). 



As the cystogenous cup grows in size, its rim begins to close 

 around the granular cell mass. This occurs after the latter has 



