Vol. 3J Torrcy. — Biological Studies on Corymorpha. 



285 



(2) The bridging cells cease to grow, according to measure- 

 ments, at al)ont the time of their fusion across the enteron. 

 Consequently, (3 and 4) any increase in the diameter of the 

 column at their level thereafter must subject them to a tension, 

 which might be relieved by cell multiplication, attenuation of 

 the bridging cells or the dislocation of some of the adjoining 

 epithelial cells which might be carried out into the bridge. The 

 facts indicate that all three methods occur. The behavior of the 

 mesogloea in Fig. 29 shows that even thus early the cells on the 

 thickened ridge are tugging at their basal ends, carrying the 

 mesogloea out into the bridge, and arranging themselves into a 

 double layer upon it, suggesting a slight evagination which is 

 not pushed out but pulled out. That attenuation of the bridg- 

 ing cells under tension may take place is shown in polyps of the 



Fig. 31. Longitudinal section through a polyp of same age as those 

 shown in Fig. 29; bridge between cavities of hydranth and stem attenuated 

 into membrane, which shows a rupture. X 460. 



same age as those represented in Fig. 29 whose diameters have 

 been augmented by internal or external pressure. Fig. 31 rep- 

 resents the partition in such a polyp, which has increased its 

 diameter because it is under strong longitudinal contraction. 

 Making allowance for the break which occurred during the 

 course of preparation, the diameter of the column at the level 

 ot the partition is almost twice as great as in Fig. 29. And as 

 the bridge or partition has lengthened it has decreased propor- 

 tionately in thickness. 



