DAVID B. SLAUTTERBACK 79 



Another pair of interstitial cells is illustrated in Figure 5. These 

 show the same complex; the absence of endoplasmic reticulum 

 membranes and now an intercellular bridge (mentioned earlier 

 today) which shows a distinct confluence of cytoplasm between 

 the conjoined cells. And as usual, there is an accumulation in the 

 extracellular space of small dense particles. They measure about 

 250 to 300 Angstroms and in all respects resemble the particulate 

 glycogen described by Fawcett and Selby in the atrial muscle of 

 turtle heart and by now in numerous other cell types. I should point 

 out, however, that it is not very common to find glycogen particles 

 extracellularly except here in the ectoderm of hydra. And in these 

 cells, glycogen, in my experience, as particulate glycogen, has never 

 been demonstrated intracellularly. Never within the interstitial cells 

 nor developing cnidoblast; only extracellularly. This would fit well 

 with the suggestion that glycogen is broken down at the cell 

 membrane. 



Returning to the intercellular bridges, your attention is direct- 

 ed to its thickened membrane which seems to impart enough rigidity 

 to the structure to resist deformation by the frequent shape changes 

 of the animal as a whole. The plasmalemma is continuous from 

 one cell to the other through the tubular bridge, although it is 

 sharply reflected upon itself twice, and bears a peculiar annular 

 expansion midway along the length of the bridge. Figure 6 is a 

 striking demonstration of this form and the continuity of cytoplasm 

 between the two cells. The vesicles in the center of the bridge 

 could hardly be said to belong to either one cell or the other. Prob- 

 ably the most important function of the intracellular bridge is to 

 synchronize differentiation and thus provide large numbers of 

 cnidoblasts in the same stage— reaching maturity at the same time. 

 But, also in the early stages of cnidoblast development, when the cell 

 is primarily concerned with proliferation, these intercellular bridges 

 undoubtedly serve to synchronize the mitoses. It is possible, with 

 some speculative stretch of the mind to suppose that the sub- 

 stance which synchronizes these mitoses must therefore be a 

 soluble substance, readily and rapidly transmitted from one cell to 

 the other. And, so we have here some evidence for the fact that the 

 nucleus when telling cytoplasm to begin a mitotic division, 

 transmits this information by some relatively small molecule, or 



