The Fine Structure 

 of Cells in Hydra 



Arthur Hess^ 



Department of Anatomy, Washington Univers^ity School of Medicine, St. Louis, 



Missouri. 



Hydra can be considered to have the following anatomical 

 regions: tentacles, hypostome or mouth region, column or stomach, 

 peduncle and basal disk. Sections of the colvmm will serve most 

 frequently to introduce the general histology of hydra. Then 

 variations of the different body regions will be presented. 



Hydra has in general two cellular layers, ectoderm or epidermis 

 and endoderm or gastrodermis separated by a layer called mesoglea. 

 The ectoderm is composed basically of epithelio-muscular cells and 

 contains dispersed cnidoblasts or nematocyst-bearing cells and 

 interstitial cells or undifferentiated cells. Gland cells occur in spe- 

 cialized regions. The endoderm contains gland cells, digestive cells 

 and interstitial cells in its generalized areas. Cnidoblasts occur only 

 rarely in the endoderm. The mesoglea is acellular. 



Whole Hydra oligactis were fixed in an extended state in 

 Dalton's fluid, a solution containing 1% osmium tetroxide, 1% 

 potassium dichromate at a pH of 7.2 to 7.6 and 0.85% sodium chlo- 

 ride, in an ice bath for 15-45 minutes. Sometimes, the Hydra was 

 divided into its various body regions; at other times, the animals 

 were treated as a whole. They were then dehydrated in alcohol and 

 embedded in methacrylate or araldite. Some sections were stained 

 with lead acetate or potassium permanganate. They were photo- 

 graphed in the electron microscope. 



iThe author wishes to acknowledge the participation of Dr. A. I. Cohen and 

 Mrs. Dorothy Sanderson in tliis study. The author's present address is Department 

 of Physiology, University of Utah College of Medicine, Salt Lake City, Utah. 



