252 THE HYDRA 



described by some investigators as divided toward their bases 

 into prop-like structures connecting the outer portion of the cell 

 with the region of the muscle processes that extend along the 

 supporting lamella. According to this interpretation, the cavities 

 that appear in the basal half of the ectoderm are, therefore, spaces 

 between the larger cells and not so much vacuoles within the cells 

 themselves as is often supposed. Partially filling these cavities, 

 are smaller cells, the interstitial cells, and their derivatives, the 

 cnidoblast cells; and, also, the nerve cells. The outer portions of 

 the cnidoblast cells, and the sensory cells of the nervous system 

 lie in the upper region of the ectoderm. If this interpretation is 

 the correct one it is the peculiar shape of the large epithelio-mus- 

 cular cells and their complex relationships with the other cells of the 

 ectoderm that render the organization of this layer so difficult to 

 demonstrate in ordinary laboratory study of sections or macera- 

 tions. The structure and functions of the nerve cells and sensory 

 cells will be considered in the account of the nervous system. 

 The development of the germ cells, which arise from interstitial 

 cells when the reproductive organs are produced, will be described 

 along with the reproductive system. There remain for considera- 

 tion here the interstitial cells and their derivatives, the cnidoblasts. 

 The Cnidoblast Cells and their Nematocysts. — Scsittered over 

 the body, but reaching their full development only upon the ten- 

 tacles, are the cnidoblast cells, containing the nematocysts, or sting- 

 ing capsules, by which the hydra paralyzes its prey (Fig. 125). 

 The relationship of these to other cells can be understood if the 

 history of the nematocysts and their cnidoblast cells is followed in 

 connection with the schematic representation in Fig. 126. While 

 it is difficult to ascertain the exact conditions in structures so mi- 

 nute, it appears that the relationships are as follows: An interstitial 

 cell (Fig. 123) produces in its cytoplasm a structure which has at 

 first the appearance of a vacuole and is later seen to be a minute 

 capsule containing a thread that becomes surrounded by a stainable 

 fluid. Apparently the thread originates as an ingrowth from one 

 side of the capsule wall, much as a bfind tube might be formed by 

 pushing in a small area at one end of a football bladder, and grad- 

 ually assumes the relationship of an elongated and turned-in 

 glove finger. The nematocyst is, therefore, not a cell, but a capsule, 

 containing a hollow inverted thread, and it is produced within the 

 cytoplasm of an interstitial cell. When an interstitial cell thus 



