240 THE BIOLOGY OF HYDRA : 1961 



It is interesting to note that all purified proteins which have 

 been assayed were inactive in this system. These include casein, 

 bovine albumin, insulin, hemoglobin, and six bovine plasma frac- 

 tions. The wide distribution of activity in crude protein fractions, 

 contrasted with the absence of detectable activity in highly purified 

 proteins, suggests that the growth-stimulating factors could be small 

 molecules bound firmly to crude protein, but removable by repeated 

 purification. The evidence at hand has not enabled us to identify 

 the Hydra growth-promoting principle with any specific previously 

 recognized growth factors for other organisms. 



While the micro-injection technique has been a very useful 

 method in the initial investigation of nutrition, it is still a tedious 

 procedure because of the individual attention required for each 

 Hydra. The mass culture of intact animals on a defined diet would 

 obviously require different methods. It appears from consideration 

 of other tissues cultured in vitro that the absolute biochemical re- 

 quirements can be determined only by study at the cellular level. 

 With the current progress toward maintaining coelenterate cells 

 in vitro, the time may be near when hydra cells may be used in 

 nutritional research. 



I think the significance of our own experiments lies not in the 

 determination of specific nutrient requirements, but in the demon- 

 stration that Hydra can live and grow on a non-living diet, and 

 that nutrition of Hydra can be studied quantitatively by its effects 

 on a measurable physiological process — namely the asexual growth 

 process. Although we have only made a start toward understand- 

 ing the nutrition of Hydra, we are encouraged to believe that it 

 is a step toward developing the full potential of hydra as an experi- 

 mental system. 



REFERENCES 



1. Ham, R. G., and R. E. Eakin. 1958. Time sequence of certain physiological 



events during regeneration in hydra. /. Exp. Zool. 139: 33-54. 



2. LooMis, W. F. 1954. Environmental factors controlling growth in hydra. /. Exp. 



Zool. 126: 223-234. 



3. LooMis, W. F. 1955. Glutathione control of the specific feeding reactions of 



hydra. Ann. N.Y. Acad. Sci. 62- 209-228. 



