OBSERVATIONS ON FRESH-WATER POLTZOA. 145 



be considered as a particular sort of bnd in which the formation of a 

 polypide remains latent until the next year. 



With regard to relations ef germinal layers in a primary C3'stid, 

 all the granular cells of the " Bildungsmasse " might with propriety 

 be called the mesoblast on grounds of their geneyis and of their future 

 history. For the same reasons, the enveloping epithelium might be 

 looked upon as the ectoblast except at the growing point, i.e. where 

 the buds are formed. At this point the cells are still in undifferen- 

 tiated embryonal condition comparable to cells of a blastula which 

 differentiates into Ectoblast and Entoblast for the first time at its 

 invagination. As the colony grows, the growing point of the pri- 

 mary cystid is split and transmitted into each succeeding bud, very 

 much like the growing point of a plant ; in other words all the 

 growing points seen in marginal polyzooids of a polyzoan colony have 

 started. I believe Bra?m is of the same opinion. Considering, on the 

 contrary, the outer layer of the ectocyst at the growing point as 

 strictly epiblastic, the conclusions, to which jSTitsche, Joliet, Salensky, 

 • &c. were led, that no hypoblast enters into the bud and that it is 

 formed as a secondary product of the epiblast, are certainly unavoid- 

 able. But such a conclusion does not accord, as was pointed out by 

 Haddon, with the generally accepted nature of budding in the animal 

 kingdom. In my opinion the budding in Polyzoa is only so far ex- 

 ceptional as the Epiblast and hypoblast take part in an undifferentiated 

 embryonal condition. 



