78 THE BIOLOGY OF HYDRA : 1961 



topic, devoting most of my time to one of the lines of differentiation 

 available to interstitial cells— the cnidoblast. Since Dr. Hess has 

 shown you excellent low power electron micrographs for orientation 

 I shall not include them in my presentation. 



The small, relatively undifferentiated interstitial cell is found in 

 the gastroderm where it gives rise (at least) to the zymogenic and 

 mucous cells, and in the ectoderm where it may differentiate 

 into cells of the gonads, cnidoblasts and possibly some others. Figure 



1 is an electron micrograph of a pair of interstitial cells in the 

 ectoderm of hydra. The nucleus is large and the nucleolus very 

 dense, but undoubtedly the most impressive feature of these cells 

 is the large number of cytoplasmic granules which are molecules of 

 ribonucleoprotein (RNP). In the cytoplasm of these cells, aside 

 from the ribonucleoprotein granules, or ribosomes, as they are 

 known to biochemists, there are no elements of the endoplasmic 

 reticulum, or at least they are very sparse. The Golgi complex is 

 represented, but only by a very few vesicles, showing a low degree 

 of organization. Another pair of interstitial cells is seen in Figures 



2 and 3. They illustrate the fact that the nuclear membrane of 

 these cells has a specialization common to many other cell types, 

 as at "Po" in the figure. These small circles which appear in a tangen- 

 tial view of the nuclear membrane, and in longitudinal sections 

 as indicated by the arrows, represent what have been called nu- 

 clear pores. Whether or not they are physiologically "pores" or 

 "holes" in the membrane, I think remains unproved. But in any 

 case, it is likely that they represent specialized areas for transmis- 

 sion of materials from nucleus to cytoplasm. This is exactly the 

 kind of thing one would like to see in a cell which is about to differ- 

 entiate, or for that matter, in a cell which is undergoing rapid 

 mitotic division. However, the great desire to believe in such 

 things, does not really substantiate their functional significance. So, 

 while they may represent the lines of communication along which 

 the nucleus tells the cytoplasm "now it's time to divide," or "now 

 it's time to differentiate," this is largely speculative. 



These pores may be seen to better advantage in Figure 4, where a 

 rather large piece of nuclear membrane has been cut in tangential 

 section. The abundance of these structures in the nuclear envelope 

 can be seen clearly. 



