Regeneration in Hydroinedusae 15 



warrant definite conclusions, still it would seem rather remark- 

 able that a critical examination of many hundreds of sections 

 should have failed to reveal the presence of mitosis if at all 

 prevalent. 



TUEULARIA CROCEA 



Distal Tentacles. — The distal tentacles seem to form about 

 as Stevens (1901) states for T. nieseinbryantheinuin. Several of 

 the entoderm cells are squeezed away from the enteric cavity into 

 the entodermal tissue, forming a sort of column as shown in figs. 

 22, 23. This causes the ectoderm to push outward slightly. 

 These entoderm cells are gradually forced further and further 

 away from the enteric cavity, sometimes assuming a position en- 

 tirely within the ectoderm, against the outer wall of the entoderm, 

 and with their long axes at right angles to the long axes of the 

 other cells. This of course pushes the ectoderm outward still 

 more, forming a ridge. The ectoderm then gradually folds 

 around this column of cells, finally enclosing it, the edges of the 

 ectodermal folds fusing and thus pinching off the tentacle. Fig. 

 24 shows a condition sometimes found ; a greater number of 

 entoderm cells are pushed outward to form the entodermal col- 

 umn, the enclosing by the ectoderm and the completion of the 

 tentacle being brought about in the usual way. 



Proximal Tentacles. — The proximal tentacles are formed 

 somewhat dift'erently, seeming to be the result of complex fold- 

 ing brought about in the following manner. The first evidence 

 of developing tentacles is a slight folding of both ectoderm and 

 entoderm (fig. 16). The folds, at first rather loose and sinuous, 

 gradually crowd closer together till the edges touch (figs. 17, 18). 

 By this time the folds have elongated radially and a sort of 

 entodermal column is thus formed, though it is not as distinctly 

 marked as in the distal tentacles. At this stage the whole region 

 of the developing tentacles is composed of a series of narrow 

 elongate folds (fig. 19). The entodermal column is composed 

 of two rows of cells with their edges closely dovetailed or else 

 flattened and the edges overlapping. XMien this stage is reached 

 the edges of the ectodermal folds begin to approach each other, 



287 



