CELL DIFFERENTIATION 



TABLE I 



Control of Sexual Differentiation in Hydra bv fCO., 



513 



flicting results that is most comforting to an experimenter. Before free 

 CO., was recognized, thirteen separate variables had had to be controlled 

 for the 2 or 3 weeks needed for such experiments. Thus, after 4 years of 

 work, I was forced to investigate carbon dioxide tension or pCO.^. How 

 rewarding this has been will appear in the account below, for it not only 

 appears that pC02 is the chief variable responsible for sexual differentiation 

 in hydra, but that this factor plays a large and varied role in many other 

 biological phenomena connected with growth and differentiation [3]. 



Let us start from the beginning. Almost all animal cells respire. 

 Crowding respiring cells together therefore induces partial anaerobiosis in 

 the centre of a cell aggregate that consists of (i) lowered oxygen tension, 

 and (2) increased PCO2. Two inverse gradients exist therefore in such a 

 cell mass, the pCO., gradient resembling the temperature of the sun in that 

 it is highest in the centre and lowest on the outside surface. Both of these 

 gradients may operate at times to create disparity within a previously 

 homogeneous group of cells. Between the two, pCO._, would seem to be the 

 more likely candidate for Aristotle's "A" because it enters into such a wide 

 variety of cellular reactions as the synthesis of purines and pyrimidines, 



VOL. 11. — 21. 



