8 THE BIOLOGY OF HYDRA : 1961 



ing huge vacuoles. The ectoderm is also very thin and has a series 

 of bulges or ridges. At the height of each ridge are present the 

 cnidocytes containing apparently mature nematocysts (Fig. 22). 

 The cnidocytes can be surrounded by the cytoplasm of epithelio- 

 muscular cells. Sometimes the cnidocytes rest on the muscle layer, 

 in which case, epithelio-muscular cell cytoplasm is on three sides 

 of them (Figs. 22, 24). At other times, the cnidocyte is the surface 

 cell (Fig. 24). However, as far as we can determine, each cni- 

 docyte is enclosed by a complete cell membrane, even when one 

 cnidocyte abuts against another (Fig. 24). Hence, the syncytial 

 relationship of cnidoblasts with immature nematocysts seen in the 

 column has broken down during the maturation of these cells seen 

 as cnidocytes in the tentacle. Frequently, muscle filaments are 

 present in the cnidocyte (Fig. 24). At what stage of cnidoblast 

 development these muscle filaments make their appearance is 

 unknown. 



The bud (Fig. 30). We have not studied the bud in detail, but 

 we have noticed that the ectodermal cells of mother and bud fuse 

 insensibly with the cuticular layer of the mother continuous over 

 the surface of the bud. The portion of bud attaching to the mother 

 Hydra looks essentially like the peduncle of the mother Hydra with 

 very vacuolated endodermal digestive cells and ectodermal epithe- 

 lio-muscular cells containing granules near their surface. 



Pertinent literature is cited in: 

 Hess, A., A. I. Cohen, and E. A. Robson. 1957. Observations on the structure 



of hydra as seen with the electron and hght microscopes. Quart. }. Microscop. 



Sci. 98: 315-326. 



EXPLANATION OF PLATES 

 All photographs are electron micrographs of Hydra. The line on the 

 photographs indicates 1/j,. 



PLATE I. Fig. 1. Cross section of the ectoderm showing the epithelio- 

 muscular cells with their nuclei (N), inclusion bodies (I) frequently found in 

 these cells, the muscle filaments at the base of the epithelio-muscular cell 

 forming the ectodermal muscular layer (E), and the vacuoles (V) in the cells. 

 M is the mesoglea. 



Fig. 2. The arrows point to the "snap-fastener" relationship between two 

 cells in the ectoderm where a portion of cytoplasm of one cell evaginates and 

 indents an adjacent cell. 



