ARTHUR HESS 3 



the surface cell of Hydra. These cells have mitochondria and a 

 Golgi apparatus. However, it is the presence of the double mem- 

 branes or endoplasmic reticulum which renders these cells distinc- 

 tive. The cnidoblasts apparently are deri\ed from interstitial cells. 

 The cnidoblasts bearing \ery immature nematocysts have a series 

 of vesicles (Fig. 7). As the nematocyst matures, these vesicles in- 

 crease in amount and extent and apparently coalesce until the sys- 

 tem of double membranes within the cell becomes quite elaborate 

 and striking ( Fig. 8 ) . The nematocyst increases in size and dis- 

 places the nucleus. In cnidoblasts with well-developed nemato- 

 cysts, the double membranes begin to decrease in amount ( Fig. 9 ) . 

 In cnidocytes having what appear to be mature nematocysts, the 

 double membrane system appears to have regressed and the cyto- 

 plasm of the cells is again granular with only a few strands of double 

 membrane remaining ( Fig. 10 ) . 



These cells are also apparently in syncytial relation to each other 

 and frequently, the cell membranes between adjacent cnidoblasts 

 can be seen to be lacking. Apparently the syncytium is no longer 

 present after the nematocysts are mature and the cnidoblasts have 

 completed their differentiation and are called cnidocytes. Each 

 mature cnidocyte appears surrounded by a complete cell membrane 

 in the tentacle, as will be shown later. 



THE MESOGLEA 



(Figs. 1, 15, 16, 19-21). The mesoglea presents a vary- 

 ing appearance in electron micrographs. It may appear fibrous or 

 granular. Some of this \ariability may be due to the state of con- 

 traction or extension of the Hydra during fixation. No cells are 

 present. Pieces of cytoplasm seen in the mesoglea can be seen to 

 be connected to ectoderm or endoderm cells which are pushing 

 into the mesoglea. These pieces of cytoplasm are surrounded by 

 a cell membrane and thus strictly are outside the mesoglea. 



The mesoglea is apparently not surrounded by its own limiting 

 membrane. It penetrates between the cells of the ectoderm and 

 endoderm (see especially Figs. 15 and 19), and granules, similar 

 to those seen in the mesoglea, can be found in extracellular spaces 

 between ectodennal and endodermal cells (Figs. 15, 19, 23, 25). 



