52 THE BIOLOGY OF HYDRA : 1961 



between muscle elements and connective tissue, or mesoglea, would 

 also be expected, and indeed they also occur. The purpose of this 

 presentation is to review some of these relationships as I have 

 observed them using light and electron microscopy. These observa- 

 tions pose a great number of additional questions which will require 

 some new approaches for further elucidation. 



In this presentation I will refer to the intercellular attachments 

 as desmosomes. I prefer desmosome as a general descriptive term 

 for intercellular attachments because the term was originally pro- 

 posed with a recognition of the functional relationship and basic 

 similarity of the various forms of intercellular attachment (9). 

 The concept of desmosome (literally "bonding body") seems to be 

 well substantiated by micromanipulation experiments with various 

 kinds of epithelium from different organisms. 



The present observations were made on specimens of Chloro- 

 hydra viridissima and Pelmafohijdra oligactis. Material was fixed 

 in osmium tetroxide buffered in acetate-veronal (6) or s-collidine 

 (1) at pH 7.4. The tissue was dehydrated in ethyl alcohol and 

 embedded in a mixture of n-butyl and methyl methacrylates or in 

 either Araldite or Epon epoxy resin (see Luft, ref. 5). Light 

 micrographs were made from one micron sections cut from epoxy 

 embedded blocks and stained according to the method of Rich- 

 ardson, et al. (7). The electron microscopy was done on an RCA- 

 2C with an improved power supply and with a Siemens Elmiskop I. 



The epithelial layers of hydra mostly consist of single layers 

 of cuboidal to columnar epithelial cells. In the epidermis interstitial 

 cells occur between the epithelial cells near their bases and nema- 

 tocytes occur between epithelial cells at the outer surface of the 

 animal. The gastroderm contains two easily identifiable cell types, 

 nutrient cells and glandular cells. A thin lamella of mesoglea sepa- 

 rates the two epithelial layers. This general configuration is dem- 

 onstrated in the first illustration. Figure la is a light micrograph of 

 a transverse section through the region of the hypostome in 

 Chlorohydra. Glandular cells and basally located intracellular 

 symbiotic Zoochlorellu may be identified in the gastrodermis. 

 Light areas near the mesoglea at the base of the epidermal cells 

 represent cross sections of muscle fibers. Figure \h shows a trans- 

 verse section through the column of Pelmatohydra. The larger 



