88 CAROLINE McGILL. 



Explanation of Plate IV. 



Abbreviations. En, endoderm cell ; ec, ectoderm cell ; /, supporting lamella ; ic, 

 interstitial cell ; nut, nutrient sphere ; nem, nematocyst ; s gl, secretion granule ; gl, 

 gland cell of the endoderm ; p, pigment ; cnid, cnidoblast cell. 



Fig. i. Cross-section of the tentacle of a Hydra exposed eight days to a tempera- 

 ture of 4 to 6° C. En, endoderm ; nut, nutrient sphere ; /, supporting lamella ; ic, 

 interstitial cell ; ec, ectoderm cell ; nem, nematocyst. 



Fig. 2. Endoderm from the body of the same Hydra as Fig. I. En, endoderm 

 cell ; gl, gland cell ; p, pigment ; nut, nutrient sphere. 



Fig. 3. Cross-section of the body of the bud of the same Hydra as Fig. I. En, 

 endoderm ; /, supporting lamella ; ec, ectoderm. 



Fig. 4. Gland cell from the foot of the Hydra described in Fig. 1. S g, secretion 

 granules. 



Fig. 5. Cross-section of the body wall of a Hydra exposed for six days to a tem- 

 perature of 4 to 6° C. This Hydra had been collected in summer from a warm 

 pond. Notice the few vacuoles in the cytoplasm. P, pigment ; en, endoderm ; /, 

 supporting lamella ; cnid, cnidoblast cell ; nem, nematocyst ; i c, interstitial cells. 



Fig. 6. Endoderm cells from the body wall of a Hydra which had been exposed 

 to a temperature of 2° C. for nine days. The structure is almost identically that of a 

 normal Hydra. Nut, nutrient sphere ; gl, gland cell ; p, pigment. 



