THE BIOLOGY OF HYDRA : 1961 



GENERAL HISTOLOGY 



THE ECTODERM 



Epithelio-muscular cells (Figs. 1, 3). Vacuolated cells are seen 

 in the ectoderm. Their nuclei are large, of even granular texture, 

 and contain prominent nucleoli. These cells have a few double mem- 

 branes and many mitochondria in their cytoplasm. They frequently 

 contain dense inclusions, which we have not as yet identified. Small 

 vacuoles, in addition to the large ones, are present. Within these 

 cells and accumulated at their base, closely packed bundles of fine 

 fibrils arranged in parallel and running longitudinally with respect 

 to the column axis are seen. The muscle system will be considered 

 separately later. 



These cells, frequently but not always, are the surface cells of 

 Hydra. The surface of Hydra is covered by a granular material 

 resting on two membranes (Fig. 6). One membrane obviously 

 belongs to the surface cells, usually epithelio-muscular, but can be, 

 at times, the cnidoblast. The other membrane apparently does not 

 belong to a cell. A short \arying distance separates the outer mem- 

 brane of the surface cell and the membrane on which the granular 

 material rests. Hydra, therefore, appears to be covered over most 

 of its surface by this thin cuticular material. 



Interstitial cells (Figs. 4, 5). Groups of small, rounded cells 

 occur in the ectoderm. They are numerous in some areas and absent 

 from others. These appear to be interstitial cells. They are charac- 

 terized by having a very finely granular particulate cytoplasm with 

 no double membranes. Mitochondria and a Golgi apparatus are 

 present. Their nuclei are evenh^ granular with one or more promi- 

 nent nucleoli. The cells are frequently very intimately related to 

 each other and at times, the limiting membranes between two adja- 

 cent cells appear to be lacking and the cells appear to be syncytial 

 (Fig. 5). Since these cells give rise to cnidoblasts, some interstitial 

 cells can be seen with a few double membranes in their cytoplasm 

 suggesting that they are beginning their differentiation. These cells 

 can be seen at times adjacent to the muscle layer on the mesoglea. 



Cnidobkists (Figs. 3, 7-10). Cnidoblasts frequently occur in 

 groups and can be found near the mesoglea or sometimes forming 



