14 George Thomas Hargitt 



gradually coming nearer and nearer (fig. 20). Finally they en- 

 tirely surround the entoderm cells and thus cut off the tentacle 

 (figs. 20, 21). 



TUBULARIA TENELLA 



Distal Tentacles. — The distal tentacles are developed about 

 as in T. crocca. The entoderm cells are forced away from the 

 enteric cavity, folding the ectoderm, which gradually encloses the 

 entoderm cells and thus forms the tentacle. Fig. 30 shows the 

 ce'ils being squeezed away from the enteric cavity. In fig. 31 later 

 stages are shown. Quite often the first indication of a develop- 

 ing tentacle is a single cell (sometimes several) lying in the ecto- 

 derm close against the supporting layer and with its long axis at 

 right angles to the long axes of the other cells. The origin of 

 this cell can not always be definitely determined from the series of 

 sections in which it occurs, though it is doubtless entodermal. In 

 other series it can be definitely traced to its original position in 

 the entoderm. In series in which the cell can not be traced from 

 the ectoderm into the entoderm, the hydranth had probably de- 

 veloped beyond the initial stage and the entoderm cell (or cells) 

 had already undergone the migration from the normal position in 

 the original stem. This single cell in the ectoderm seems to go 

 through repeated divisions, forming a mass of cells, after which 

 the development proceeds in the manner already described. 



Proximal Tentacles. — The formation of the proximal ten- 

 tacles is quite different from that of T. crocca, being accom- 

 plished by evagination in the early stages and not by folding. 

 The evagination begins in the entoderm, the ectoderm being sim- 

 ply pushed outward. Figs. 25 to 28 show the course of de- 

 velopment. In fig. 25 the entoderm has begun to evaginate. push- 

 ing the ectoderm outward slightly. In fig. 26 some of the ento- 

 derm cells have been pushed away from the enteric cavity and 

 the folding of the ectoderm continued. Fig. 27 shows the evagin- 

 ation nearly completed and the cells arranged in two rows, with 

 edges interlocking. At about this stage the ectoderm begins to 

 surround the evaginated entoderm cells, continuing until the folds 

 of ectoderm meet, fuse, and thus separate the tentacles as shown 

 in fig 28. In longitudinal sections the evagination is even more 



288 



