400 THE BIOLOGY OF HYDRA : 1691 



our stock clone when compared to previous responses which the 

 organisms had given. Also, we were continually confronted with 

 daily erratic behavior in Hydra taken from the same culture dish 

 and treated in what we believed to be an identical fashion. And 

 finally, it was found that our stock clones were uniformly parasitized 

 by an intracellular protozoan, a species of Microsporidia. It was this 

 point that we were ready to throw all our clones back into the lily 

 pond from whence their great grandparent had come. In our earlier 

 studies, we used one strain of Hydra and indeed were apprehensive 

 about bringing into the laboratory any other strain of Hydra littor- 

 alis, lest there be mixing of stock clones. However, because of our 

 difficulties we did bring other strains of //. littoralis into our labora- 

 tory — retreated, as it were, in our program— to re-examine more 

 critically factors which could be causing variations in the response 

 of "normal" untreated hydra. 



Although it was a laborious undertaking, it pro\'ed worthwhile 

 in that it not only pointed out the causes of the inconsistent results 

 we had been getting, but also increased by a considerable extent 

 our knowledge concerning both extrinsic and intrinsic factors 

 which influence regeneration. This new data, in turn, led to histologi- 

 cal studies giving valuable information concerning some of the basic 

 factors influencing the rate and extent of regeneration and enabling 

 IIS to develop a hypothesis for use in planning future investigations. 



For the purpose of discussion, the factors studied can be classi- 

 fied as (A) intrinsic factors and (B) extrinsic factors. 



In selecting criteria for measuring regenerative capacities of 

 hydra, we have used two measurements which can be rapidly 

 determined on a large number of organisms — namely, (a) the 

 length: width ratio observed for the longest developing tentacle 

 formed during the early stages (18th to 24th hour) of regeneration, 

 and ( b ) the total number of tentacles observed after the regenerative 

 process is well along ( after the 44th hour ) . The early measurement 

 of the extent of the growth of the future tentacle gives some indica- 

 tion of the rate at which morphological processes can be initiated. 

 The total number of tentacles regenerated expresses roughly the 

 total amount of morphogenic change. These two determinations ob- 

 viously are a measure of the composite effects of many individual 

 factors, but they have permitted us to run screening tests on thou- 



