Regeneration in Hydromedusae 15 



distinctly shown ( fig\ 29). The development thus far has taken 

 place within the perisarc as in T. crocea. When the regenerating 

 hydranth emerges from the old perisarc the tentacles are not, as 

 in T. crocea, fully formed (cf. figs. 14^ to 14/), but only attain 

 their final length by further growth. This would seem to indi- 

 cate very clearly that, although in the beginning the forniation of 

 tentacles may be by the transformation 01 old tissue, the comple- 

 tion of the process is by new growth. Bickford's conclusions, al- 

 ready referred to, may therefore be true for the body of the 

 hydranth, but the tentacles are at least partially of new growth. 



TUBULARIA LARYNX 



Distal Tentacles. — The distal tentacles form in about the 

 same manner as in T. tenella. The single cells lying in the ecto- 

 derm, against the supporting layer, are found somewhat more 

 commonly than in T. tenella and it is more difficult to trace their 

 origin, though they are doubtless entodermal. However, in sev- 

 eral series of sections this cell has been traced to a position partly 

 in the ectoderm and partly in the entoderm (fig. 43). In a num- 

 ber of series, also, entodermal cells seem to have been forced 

 away from the enteric cavity in a manner similar to that found in 

 7". tenella (figs. 36 to 38). In these figures the appearance of 

 earlv stages is shown. These cells being forced away from the 

 enteric cavity more and more form a column of cells as shown 

 in figs 39. 40. The ectoderm gradually surrounds this column 

 and thus the tentacle is formed. Fig. 43 shows the single cells 

 in the ectoderm. These cells seem to divide and form a column 

 within the ectodermal layer (fig. 41), and are gradually sur- 

 rounded by the ectoderm and the tentacle pinched olT (fig. 42). 



A comparison of figs 41 to 43 with figs. 36, 39, 40, suggests 

 the possibility that in some cases the first cell squeezed away from 

 the enteric cavity is forced into the ectoderm immediately and 

 by division forms a column of cells, which is surrounded by the 

 ectoderm to form the tentacle ; w-hile in other cases the necessary 

 number of entoderm cells are gradually forced away from the 

 enteric cavity into the ectoderm and without undergoing any 

 divisions make up the core of the tentacle, which is surrounded 

 by the ectoderm in the usual way. 



289 



