420 THE BIOLOGY OF HYDRA : 1961 



contact. Gastrodermis removal and fasting studies have indicated 

 that a normal amount of gastrodermis and proper nutrition are 

 necessary for normal regeneration. It is possible that a transfer of 

 typical nutrients, or possibly some other specific substance neces- 

 sary for epidermal interstitial cell differentiation, might be better 

 facilitated when the two cellular layers are in closer contact. If 

 this is the case, then variations in the amount of mesogloea might 

 be an important factor in growth regulation of hydra by in- 

 fluencing the extent of interstitial cell differentiation. 



Induction of the differentiation of one tissue by its close contact 

 with another tissue has been studied extensively, but the exact 

 mechanism is not known. Although it is interesting to compare 

 the situation in hydra (wherein close contact of gastrodermis and 

 epidermis occurs during active interstitial cell differentiation) to 

 induction in embryological studies, there is no direct evidence that 

 the mechanism is the same. However, Moore (8), working with 

 the hydroid Coidijloplwrci lacustris, noted that oral cone grafts 

 will induce from mass tissue the development of hydranth regions 

 basal to the oral cone. She observed that no induction was pro- 

 duced when an oral cone graft was separated from the host tissue 

 by an agar or a cigarette paper barrier. "Direct close contact 

 between the graft and the host appears to be necessary for induc- 

 tion to be produced." 



The origin of mesogloeal material during regeneration is not 

 known. In regenerating hydra, the mesogloea is restored in the 

 hypostomal region, usually by 18 hours regeneration time, after 

 interstitial cell differentiation has occurred. That the mechanisms 

 involved in restoration of the mesogloea are not identical with 

 those involved in interstitial cell differentiation is apparent in 

 lipoic acid treated Hydra littoralis, wherein interstitial cell 

 differentiation has not occurred normally by 18 hours, yet the 

 mesogloea is restored at the same rate as in normally regenerating 

 controls. If one considers that a normal balance must occur be- 

 tween the rate of interstitial cell differentiation and mesogloeal 

 restoration, then a growth regulating mechanism can be postulated 

 wherein alteration of either of these processes can cause a variation 

 in the normal regeneration pattern. 



