DOROTHY B. SPANGENBERG 419 



INTERACTION OF CELL LAYERS 



The two cellular layers of hydra, the epidermis and gastroder- 

 mis are separated by a non-cellular substance, the mesogloea, 

 which varies in thickness throughout the column of the hydra. 

 The mesogloeal layer is extremely thin in normal hydra at the 

 tentacle base and at the site of a newly forming bud. In regenerating 

 hydra, the mesogloea is not replaced at the regenerating tip until 

 the early tentacles have appeared and the hypostome has formed. 

 In lipoic acid treated Chlorohijdra, areas which are lumpy in form 

 show a decrease or absence of mesogloea ( 13 ) ; and according to 

 Chang, Hsieh, and Liu (3), in depressed hydra, prior to develop- 

 ment of abnormal form, the mesogloea "dissolves." In all of 

 these instances, in areas where there is active differentiation of 

 interstitial cells prior to normal or abnormal growth of tissue, the 

 mesogloea appears reduced or absent. Conversely, at the tentacle 

 bases of fasted hydra and in the stalk region of normal hydra, 

 where there is little interstitial cell differentiation taking place, the 

 mesogloea appears to be relatively thick. In several hydra which 

 have been permanently inhibited by lipoic acid, at 48 hours after 

 cutting, the mesogloea has been restored although the hydra have 

 not regenerated. 



The foregoing does not imply that interstitial cell differentia- 

 tion takes place only in areas where there is no visible mesogloea. 

 A certain amount of interstitital cell differentiation is undoubtedly 

 taking place continually in areas that contain visible mesogloea. 

 However, we believe that the differentiation process is hastened 

 in areas of active growth by a closer contract of the two tissue 

 layers in the absence of visible mesogloea. 



Whether the mesogloeal material is actually utilized in areas of 

 active cellular differentiation is not known. If the mesogloea does 

 contain collagen or a collagen-like material as postulated (12), 

 then it is possible that this substance may be utilized in a manner 

 analagous to its use in wound healing in higher animals. Whether 

 or not such use is made remains to be determined. 



Where the mesogloea is not seen in various areas of the hydra, 

 however, the gastrodermis and epidermis appear to be in closer 



