132 THE BIOLOGY OF HYDRA : 1961 



deuce exists which suggests that the new findings are generally 

 applicable to problems of cell growth and cell differentiation. 



The present report will be limited to some recent observations 

 pertaining to the structure of stenoteles, the largest of the four types 

 of nematocysts of Hydra. 



MATERIALS AND METHODS 



Entire Hydra (H. vulgaris and H. littoralis) were fixed for 1 to 

 2 hours at 2 to 5 in 1 per cent osmium tetroxide buffered to a 

 pH of 7.6 with acetate-veronal buffer. The fixative also contained 

 about 0.7 per cent sodium chloride and a few drops of 0.11 M 

 calcium chloride. 



Following fixation, the specimens were washed, dehydrated 

 through a graded ethyl alcohol series and were embedded in Epon 

 according to a schedule developed in Dr. H. Stanley Bennett's 

 laboratory by Drs. J. H. Luft and R. L. Wood (1). 



Sections were cut with a Servall Porter-Blum ultramicrotome 

 using glass knives prepared in the laboratory. The sections were 

 floated onto distilled water and were picked up on collodion-coated 

 grids (Fullam #2001). Staining with uranyl acetate for 1 to 

 2 hours was carried out according to the method devised by 

 Watson ( 14 ) . Electron microscopy was accomplished with an RCA 

 electron microscope, type EMU-2D, equipped with a 0.015 inch 

 externally centerable (Canalco) platinum condenser aperture and 

 a 50^ copper objective aperture in the standard objective pole 

 piece. 



OBSERVATIONS AND DISCUSSION 



The present studies of stenotele fine structure have revealed 

 several interesting new features. Figure 1, a sagittal section of a very 

 nearly mature stenotele, shows, in addition to the structures previ- 

 ously described by Chapman and Tilney (1959b), viz., capsule (C), 

 operculum (O), invaginated capsular wall (ICW), stylets (ST), 

 spines (S), matrix (M) and tubules (T), what appears to be a 

 most fortuitous section through the enlarged "head " of the tubule 



