234 THE BIOLOGY OF HYDRA : 1961 



chemical study of development and differentiation processes — a 

 field to be discussed by Dr. Eakin. Since the nutritional state of 

 an animal affects all of its physiological processes to some extent, 

 it is desirable to be able to control the nutritional state during the 

 study of other physiological phenomena. The development of chem- 

 ically defined nutrient preparations in which cultures of hydra or 

 hydra cells could be grown aseptically would give the investigator 

 complete biochemical control over the organisms. 



Our ultimate goal was the propagation of hydra cells in a 

 chemically defined medium. With this sytem, we should be able 

 to determine the role of each tissue layer in processes such as 

 regeneration and sexual differentiation. However, we chose to begin 

 our experimentation with whole animals for two reasons: in general, 

 the requirements for cell propagation are much more critical than 

 those for growth of the intact organism. In the intact animal, speci- 

 fic trace nutrients may be supplied by specialized cells. There is also 

 a more rapid loss of essential nutrilites to the external solution 

 from the isolated cell. We hoped to discover approximate require- 

 ments before proceeding to precise studies at the cellular level. 

 Secondly, techniques for quantitative study had already been de- 

 veloped for whole hydra but not for dissociated cells. Thus the 

 nutritional value of an experimental diet could be determined by 

 its effect on an observable physiological process such as asexual 

 growth. 



Our stock Hydra clone was obtained from a locally-isolated 

 strain of Hydra littoralis, and was grown according to the methods 

 of Ham and Eakin ( 1 ) , When fed daily with an excess of freshly- 

 hatched Artemia larvae, the Hydra grows at a maximum logarith- 

 mic rate. Presumably the animal receives an excess of all exogenous 

 requirements, and the limitation of growth rate is due to necessary 

 metabolic conversions within the cells. If the exogenous supply of 

 a growth factor is reduced below the maximum utilizable by the 

 animal, a reduction in the observed rate of growth should result. 



In the search for a non-living diet, it was found that heat- 

 killed Artemia would support asexual growth of Hydra for at 

 least six months, but at a rate significantly below maximum. Al- 

 though the killed Artemia contained adequate amounts of reduced 

 glutathione to stimulate the feeding reaction, the solution had to be 



