338 THE BIOLOGY OF HYDRA : 1961 



way over and above their usual differentiation into nematocysts, 

 the phenomenon is reversible and sexual Hydra may be obtained 

 from asexual and vice versa. These various factors combined have 

 made the following study experimentally feasible. 



Since several years' work will be reviewed in the next half hour, 

 permit me to use an analogy to illustrate some otherwise confusing 

 relationships. The analogy concerns a man who wears a little woolly 

 sweater. Inside his skin we know the temperature to be 98.6 F. 

 while the temperature of the room is perhaps 50 °F. Now the 

 question is: What is the temperature to which his skin is exposed? 

 Clearly the sweater markedly affects the answer, so that the air 

 in contact with his skin is nearer 98.6 the thicker, and more 

 impermeable the sweater. How does this relate to Hydra? 



Figure 1 is a photograph of some Hydra in a Petri dish in which 

 a pH sensitive dye (brom cresol purple) is present as well as 0.5% 

 agar. This is a small amount of agar, enough to increase the vis- 

 cosity of the culture solution^ without making it actually gel. Ob- 

 serve that each Hydra is surrounded by a halo of its own making, 

 an area of increased acidity due to the increased pCOo adjacent 

 to its body surface. Each Hydra, in other words, is inside a little 

 woolly sweater, where the partial pressure or pCO:.. of carbon diox- 

 ide is neither as high as it is in his tissues proper, nor as low as it 

 is in the general macroenvironment of the Petri dish. This "halo 

 zone" corresponds then to the area inside the man's sweater. It is 

 the zone of partial anaerobiosis where the pCOo and pNHs are in- 

 creased and the pO^ and pH are decreased in a microenvironment 

 that is chemically quite different from that of the macroenvironment 

 of the Petri dish proper. 



Note that the halo zone around each individual Hydra varies 

 with the size of the Hydra, so that larger and older Hydra are 

 exposed to greater degrees of anaerobiosis than smaller and younger 

 ones. In addition, group effects are present around Hydra that 

 happen to lie close together so that their halo zones overlap and 

 mutually reinforce each other. This group effect is clearly visible 



iBVC solution composed of 100 mg./l. NaHCOa, 50 mg./l. disodiimi ediylenedia- 

 mine tetraacetate ('"Versene") and 100 mg./l. CaCL., dissolved in deionized water 

 from a Barnstead Bantam Demineralizer equipped with a red-cap Mixed Resin 

 cartridge. 



