12 George Thomas Haro^itt 



V. Histology 



In her work on Tiibitlaria tcneUa Bickford (1894, p. 422) 

 says, "In the case of this hydroicl . . . the regeneration ap- 

 pears to be largel}' a direct transformation of a stem portion 

 over into the body portion of the new hydranth." Since in her 

 experiments no definite histological demonstration of this was 

 undertaken, it has seemed desirable to repeat her experiments and 

 to work out critically the histology of the processes involved. 

 Also to determine to w'hat extent the same processes of trans- 

 formation are operative in T. crocca and T. larynx. 



Stevens (1901), who has worked out the histological changes 

 involved in the development of the tentacles in T. mcscnibryan- 

 thonnin, states that the proximal tentacles form by a folding of 

 both ectoderm and entoderm, and the distal tentacles by a rod or 

 column of entoderm being separated from the rest of the ento- 

 dermal tissue, and the ectoderm folding around it and pinching 

 off the tentacle. In regard to the increase in surface of the ec- 

 toderm made necessary by this process of folding, she says, "The 

 increase in surface of the ectoderm is due partly to cell division 

 and partly to a change in the cells from a more to a less col- 

 umnar form. Her drawings show mitotic figures in both ecto- 

 derm and entoderm cells. This does not seem to be the case in 

 Tubularia crocea, T. tencUa, and T. larynx. The cells do some- 

 times lose their typical columnar form, but not to any great ex- 

 tent. Therefore there must be cell division to account for this in- 

 crease in surface of the ectoderm. Special attention was directed 

 toward the staining and to the observation of mitotic figures, and 

 yet in no case was the slightest evidence of mitotic division pre- 

 sented. Nuclei were found which gave evidence of amitotic 

 division, and the various stages in this process were also pre- 

 sented, so that the cells probably divide directly. However, the 

 resting nuclei very commonly contain two nucleoli, and it is hard 

 to distinguish between this resting condition and the early stages 

 of amitosis. So it was not possible to prove definitely the pres- 

 ence of amitosis in all cases. 



While this is a somewhat unusual histologic phenomenon, and 

 the. wholly negative character of the evidence is insufficient to 



286 



