362 THE BIOLOGY OF HYDRA : imi 



has to keep the Krebs cycle operating by replacing the di- or tricar- 

 boxyllic acids. This is usually taken care of by the Wood-Werkman, 

 Utter, or malic enzyme pathways, all of which require a fomi of 

 pyruvate and COo. In uncrowded cultures, however, hydra, as well 

 as bacteria and tissue culture cells, probably lose much of their 

 metabolic COo to the environment. It would seem that these cells 

 would have difficulty in resynthesizing the dicarboxyllic acids unless 

 the partial pressure of CO2 was increased either naturally, as in 

 crowding, or artificially using known gas mixtures. 



If this were true, then by exposing starved H. littoralis to C^^O., 

 one might expect the C^^ to be found mostly in glutamate and 

 aspartate. This proved to be the case (Lenhoft, H., 1959, Exptl. 

 Cell Research 17, 570-574). Alanine, which comes from pyruvate, 

 and therefore would not be expected to incorporate C^^Oo, was not 

 labeled. Furthermore, the C^^ was localized in cnidoblasts, which 

 are known to be active in the synthesis of nematocyst protein in the 

 starved animal. The large amount of RNA-rich endoplasmic reti- 

 culum as shown in Slautterback's electron micrographs of cnido- 

 blasts, is another indication of protein synthesis. Also, when we in- 

 duced sexual difterentiation in starved Hydra in the presence of 

 C^^Oj, much of the C^^ was concentrated in the testes and the 

 ovaries of sexual Hydra. 



Thus, the partial pressure of CO- may take on special impor- 

 tance in animals such as hydra that readily lose COo to the en- 

 vironment. The increased pCOo would serve to drive the reactions 

 forming the dicarboxyllic acids, thereby maintaining the continued 

 operation of the Krebs cycle and thus regulating the activities of 

 the cell. 



LOOMIS: Yes, your radioautographs are very dramatic, showing 

 how the C^^ is concentrated in the growing testes and ovaries as 

 well as in the cnidoblasts. 



Added in proof: Two recent articles on pCOs are: Goddard, D. R. 1960. The 

 biological role of carbon dioxide. Anesthesiology, 21: 587-596, and Loomis, \V. F. 

 1961. Cell differentiation: a problem in selective gene activation through self- 

 produced micro-environmental differences of carbon dioxide tension, in Biological 

 Structure and Function, First lUB/IUBS Joint Symposium, O. Lindberg and T. W. 

 Goodwin, eds.. Academic Press, London (in press). 



