W. F. LOOMIS 359 



dropping but when winter anaerobiosis in the pond is steadily 

 increasing (9). 



Seen from this angle, a Hydra is nearly as naked as a clone of 

 cells growing in tissue culture. Having no perisarc, it is exposed 

 to its external milieu in feedback fashion just as is a pseudo- 

 plasmodium or a developing frog egg. In each of these cases, 

 morphogenetic gradients form and gradually shape the once- 

 identical cells into diverse populations of differentiated cells. The 

 purpose of this paper has been to show how these gradients may 

 be approached both conceptually and experimentally. 



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