W. F. LOOMIS 341 



The third addition represents the cell membrane barrier, an 

 addition that is very small in the case of COo and NH3 as the lipid 

 cell membrane is known to be highly permeable to both these dis- 

 solved gases, (it is almost impermeable to the HCO.,^ and NH4+ 

 ions that are fat insoluble ) ( 10,27 ) . For present purposes, this 

 third or membrane effect may be neglected. 



The fourth and final addition represents the intracellular pC02 

 gradient that varies both with q, the respiratory rate, and r the 

 radius of the cell. Since cell division mechanisms keep r reason- 

 ably constant, we can experimentally control this fourth factor by 

 controlling q with a thermostat, for it has been shown that the 

 respiratory rate of Hydra varies logarithmically with the tempera- 

 ture, as well as also varying somewhat with the level of nutri- 

 tion (11). 



The main factors that control the pCO^ in the center of a cell 

 according to Rachevsky are then: 1) the external macroenviron- 

 mental background; 2) the "halo zone" microenvironment; 3) the 

 barrier effect that is small if only a cell membrane is involved but 

 can be very large if it involves an impermeable chitinous perisarc; 

 and 4 ) the internal gradient. 



This then was the thinking behind the various experiments re- 

 ported below, experiments in which we studied the effects of tem- 

 perature, rate of feeding, population density, stagnation, degree 

 of aeration etc. on the sexual maturation of Hydra. It was our 

 assumption that DNA in the nucleus of the interstitial cells in the 

 hypostome can produce RNA and specific proteins such that gon- 

 adal tissues form whenever their "programming" is correct in respect 

 to such feedback variables as pH, pOo, pNHo and pCOo etc. When- 

 ever the programming is not of this variety, then these same inter- 

 stitial cells differentiate into nematocysts due to the intrinsic pro- 

 gramming that, in this case, takes place wholly within the tissues 

 of Hydra. Only in the case of sexual differentiation does the external 

 halo, group, and background effects determine the outcome of the 

 experiment. Only this case, therefore, can be experimentally manip- 

 ulated by varying the external cultural conditions. 



Let us examine the results of the experiment in Table 1 from this 

 point of view. This experiment was originally performed in 1957 

 (16) but has been repeated eight times since then with entirely 



