54 THE BIOLOGY OF HYDRA : 1961 



size and lack of Zoochlorella make Pelmotohi/dra easier to ex- 

 amine. At the free outer surface, adjacent epidermal cells are 

 bound together by terminal bar type desmosomes ( arrows ) . These 

 desmosomes were not described in earlier light microscope studies 

 of hydra or in more recent electron microscope studies by other 

 workers (2, 4, 11). Interstitial cells, nematoblasts and gastroderm- 

 al nutrient cells are seen clearly in Figure lb. 



These general features of hydra epithelia are shown to even 

 better advantage in low magnification electron micrographs. Figure 

 2 is an electron micrograph of a section through the gastric region 

 of Chlorohijdra. The prominent dense bodies in the gastro- 

 dermal cells are Zoochlorella. Other identifiable features include 

 nuclei, microvilli, other cellular inclusions and muscle processes. 

 The two epithelial layers are separated by the thin mesolamella 

 which appears dense in this picture. Desmosomes appear as areas 

 of increased density between adjacent cell surfaces, especially near 

 the outer surface of epidermal cells and the lumenal surface of 

 gastrodermal cells (arrows). Similar densities occur between 

 adjacent membranes of interdigitated muscle processes (Fig. 2, 

 circle ) . 



In both species of hydra examined the desmosomes which are 

 present near the free surfaces of epidermal and gastrodermal cells 

 display a very complex morphology when viewed at higher magni- 

 fication. The two apposed plasma membranes each exhibit the 

 dual profile of the "unit membrane" of Robertson (8), the two 

 peaks of density being about 70 Angstrom units apart. The increase 

 in density noted by light microscopy and in lower magnification 

 electron micrographs is seen to be due to a condensation of intracel- 

 lular material and to the presence of a specially oriented intercel- 

 lular matrix. These features are shown in Figure 3, a and b, an 

 example of an epidermal desmosome in a specimen prepared in 

 the usual way and then stained with phosphotungstic acid prior to 

 embedding. In this preparation the junction of at least three differ- 

 ent epidermal cells is represented. The condensation of intracellular 

 material appears somewhat vague at this magnification but the 

 organization of intercellular material is well demonstrated. The 

 two apposed cell surfaces are connected directly by a series of 

 parallel densities oriented perpendicular to the plane of the plasma 



