DIFFERENTIAL INHIBITION IN OBELIA 



483 



in the endoderm axis of a tentacle as one approaches the distal 

 extremity. This loss of differentiation gradually extends 

 proximally, and the whole tentacle passes into the gastro- 

 vascular cavity after rupture of the mesoglaea (confirming 

 Thacher, 1903). In PI. 26, fig. 4, the cavity is seen to be filled 

 with cells and debris, the result of resorption of the tentacles. 



Text-fig. 4. 



O o 



O o 



Campanulana. The distal part of the zooid has separated from the 

 proximal part. Active cilia occur in both parts, also immigrated 

 cells, but those in the proximal part are sparser as they are 

 migratmg down the stem. Some cells of the distal part are 

 migrating outwards. (Camera lucida.) 



Eesorbed elements from the hypostome are no doubt there 

 also, although indistinguishable. 



Examined under high powers, it appears that the boundaries 

 of the loose cells in the gastrovascular cavity lose more of 

 their distinctness the farther the elements are from the 

 periphery. The spherical or ovoid bodies to be seen in the 

 centre of the zooids containing refractive matter (but not 

 staining very definitely) are nematocysts. Nearer the periphery 



