340 



THE BIOLOGY OF HYDRA : 1961 



tioii of this metabolite at the center of the cell would be the sum 

 of four factors. 



At the bottom would be the macroenvironmental background, 

 which in the case of pCO^ is 0.03^i atmosphere (0.22 mm. Hg) in 

 all samples of aerated water but 5.3% atm. in mammalian blood. 



H,pNH3, pC02,etc. 



Fig. 2. Rachevsky's graph of the four zones that together determine 

 the final degree of anaerobiosis to which the DNA in the nucleus of a 

 cell will be exposed. This same analysis holds for a multicellular mass of 

 cells such as a slime mold pseudoplasmodium. Hydra, or developing frog egg. 

 See Rachevsky (25) for mathematical equation that determines the profile 

 of this graph 



Both of these backgrounds remain constant because the percentage 

 COo in the air (0.03%) is extremely constant while the pCO^ of the 

 blood is homeostatically regulated by the medullary center of the 

 brain. 



Above the background zone in Rachevsky's graph is seen the 

 halo zone referred to above. This is the external gradient that forms 

 around any respiring cell under stagnant conditions. It reflects 

 both stagnation and crowding for the group effects mentioned above 

 also increase as population density increases. 



