84 CAROLINE McGILL. 



almost identically that of a normal Hydra, except that the 

 vacuoles in the cytoplasm are not quite so large and the nutrient 

 spheres are not so numerous. The distended condition of the 

 gland cells is worthy of note. This Hydra has practically the 

 same structure throughout that one deprived entirely of food for 

 an equal length of time would have. Here, then it is impossible 

 to say whether there are any changes whatever due directly to 

 reduction of temperature. In this Hydra there are, at any rate, 

 no indications of Greely's reversal of vital phenomena. Figs. 1—4 

 are from sections of a winter Hydra after being kept eight days 

 at a temperature of 4 to 6° C. Fig. 1 shows the entire thick- 

 ness of the wall of the tentacle. The protoplasm of both endo- 

 derm and ectoderm cells is less vacuolated than in the normal 

 Hydra and the nuclei, especially those of the interstitial cells are 

 smaller and more deeply staining. In Fig. 2 the differentiation 

 of the endoderm of the body into endoderm cells and gland cells 

 is apparent. Even in the bud of this Hydra the cells are still 

 intact. Fig. 4 shows a foot cell still filled with secretion. The 

 sections of this Hydra show no changes that might not be 

 directly due to loss of water. 



In a Hydra collected in summer and placed at a temperature 

 of 4 to 6° C. for six days the cytological changes are much 

 more pronounced than in the two winter Hydra just described 

 (see Fig. 5). Here the cells are much smaller than normal, the 

 protoplasm is free from vacuoles, gland cells cannot be distin- 

 guished in the endoderm, and the nuclei of all the cells are very 

 small and deeply staining. Many of the nuclei, especially those 

 of the interstitial cells, look as if the nuclear sap had been almost 

 entirely extracted leaving only the much condensed chromatin 

 within. In a few places in this Hydra, most apparent at the tip 

 of the tentacles, the cell boundaries are indistinct. Where such 

 is the case the nuclei always are beginning to degenerate. 



In Hydra like the one described in experiment 5, where, after 

 exposure to cold, the body practically loses power of movement 

 even after the temperature is raised, the cell structure is often 

 entirely obliterated. The nuclei are in such cases always pyc- 

 notic and both nuclei and cytoplasm show every evidence of degen- 

 eration. Since it is seldom that Hydra are found which behave in 



