55 



cles and sense-organs iiake their appearance in the adult, 

 as previously described, makes it possible to locate with 

 coniparitive certainty the beginnings of one of these organs 

 on the bell-margin. Figure 18 is from a section of a medu- 

 sa, cut horizontally at the point of origin of one of the 

 tentacles. The figure shows tlie aspect at the level of the 

 tentacle, somewhat above the velum. Both cell-layers are 

 seen to be concerned in the formation of the new tentacle. 

 The endoderm (End. ) is pushed out from the region of the 

 circular canal, and has the shape of a solid plug of tissue 

 composed of a few cells arranged radially about a central 

 axis ( T.R,); the nuclei are at the inner ends of the cells. 

 Outside of this endodermal core is the ectoderm (Ect.) 

 which is, in the region of the bell-margin, of the charac- 

 ter of gelatinous tissue, stiffened by the presence of a 

 large number of concretions (Cor). These concretions and 

 the nuclei of the cells are more numerous at the point 

 where the tentacle is to appear than elsewhere. In the me- 

 dusa, as in the polyp, the greatest activity in the forma- 

 tion of a new tentacle is manifested by the endoderm. Ac- 

 cording to Allman (Monograph, 1871), in some hydroids ( Cam - 

 panularia Johnstoni . for example) the first indication of 



