ISO W. A. KEENER AND P. N. JESTER. 



an endodermal epithelio-muscular cell, does not present a 

 markedly unusual appearance. In Fig. 2, however, the enclosed 

 thread of the nematocyst displays marked shortening and the 

 nematocyst, as a whole, takes the stain less readily. In Fig. 3, 

 we have a contrast presented between a nematocyst that has been 

 almost digested and one lying in the usual position in the ecto- 

 derm. It is seen here that the nematocyst within the endodermal 

 cell has been almost wholly dissolved. Eventually no trace of the 

 ingested nematocyst is to be found. The hungry Hydra, there- 

 fore, consumes not only the readily digestible tissues of its 

 tentacles but also the nematocysts. From the material, thus 

 obtained, energy is derived upon which the life of the polyp is 

 tided through a period of inanition and by which the polyp is 

 enabled to develop a new complement of tentacles. 



An interesting feature of this process is presented in the 

 fact that the stumps of tentacles receive relatively much more of 

 the ingested tentacular material than do other parts of the body 

 Our slide "375 " shows this clearly. This slide bears in series the 

 sections of an Hydra that had been isolated in a Petri dish in the 

 laboratory for eighteen hours. At the end of this period but 

 small stumps of its tentacles had remained. In this series of 

 transverse sections there are ninety-five sections. 



The tentacles involve about twenty of these (10 microns) 

 transverse sections. The endodermal cells in the sections that 

 show the bases of the tentacle-stumps are heavily charged with 

 food-vacuoles that contain nematocysts and hydra-cells. The 

 endodermal cells of the tentacle-stumps have in this instance 

 much more food supplied them than do the endodermal cells of 

 the body proper. This is in contrast to what ordinarily takes 

 place. If a complete Hydra be fed and later sectioned, the 

 endodermal cells of the tentacles will be found to have relatively 

 fewer food-vacuoles than the endodermal cells of the body- 

 proper. So here again we have it suggested that in a diploblastic 

 animal local needs must be locally met. In Hydra, having but 

 two tissues there can be no circulatory medium. Therefore, 

 when material is needed for the regeneration of tentacles it 

 cannot be taken up by the general endoderm of the body-proper 

 and then as lymph or plasma sent to the tentacular bases, but the 



