W. F. LOOMIS 



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in Figure 1. It corresponds in our temperature analogy to the 

 warmth generated by a group of baby birds that huddle together 

 in the nest so that they create a microenvironment far warmer than 

 the surrounding air. 



Figure 2 represents Rachevsky's formulation of such a halo 

 zone ( 25 ) . He postulated that if a spherical cell of radius r should 

 give off any metabolite such as CO^ at a rate q, then the concentra- 



Fig. 1. Halo zones of partial anaerobiosis around single Hydra. These 

 vary in size with the size of the Hydra as well as with the closeness of ad- 

 jacent Hydra. See text for details. 



