358 THE BIOLOGY OF HYDRA : 1961 



cellulose deposition and hence stalk formation in all cells 

 that become buried in the central mass (see 26). Such 

 a mechanism would explain how identical slime mold 

 amoebae can differentiate into completely different types 

 of cells once they have aggregated into a single multi- 

 cellular mass whose central or medullary pCOo and pH 

 are utterly different from what they are in the peripheral 

 cortex. 

 Returning now to Rachevsky's formulation, we find that it 

 applies to both isolated single cells ( as he originally meant it to be ) 

 and to multicellular masses of cells such as hydroids and slime mold 

 pseudoplasmodia. In fact, the great permeability of cell membranes 

 to both NH3 and CO^ serves to unite such multicellular masses into 

 one supercellular field or gradient of pCOo and pNH.j. Goldschmidt 

 has stated (8) that "the most difficult and most neglected of all 

 basic fields of morphogenesis is that of supercellular integration," 

 and it is of interest therefore that Rachevsky's ideal formulation was 

 developed for an isolated cell but fits the facts in differentiating 

 metazoan tissues as well. 



For example, Rachevsky's graph clearly brings out the great 

 morphogenetic importance of a perisarc, that chitinous non-living 

 envelope that surrounds the living coenosarc of some hydroids ( 29 ) . 

 In our analogy, a perisarc would correspond, not to a removable 

 woolly sweater, but to a permanent coat of fur as seen in northern 

 animals. Both types of insulation serve to build up the internal 

 temperature gradient, but a perisarc is permanent while the halo 

 zone "woolly sweater" is dependent on the presence of stagnant 

 conditions. 



In this connection, Cordijlophora possess a perisarc and differen- 

 tiate sexually in May and June even while growing in the fully 

 aerated spillway of Nye's Pond near Woods Hole.^ In this case, 

 the perisarc probably insulates the animal sufficiently to allow par- 

 tially anaerobic conditions to form inside its tissues as the tempera- 

 ture and food supply gradually increase their metabolic rate in 

 late spring. In contrast to this. Hydra have no perisarc and turn 

 sexual in the late fall when the temperature and food supply are 



■^observed to be sexual in 1960 by C. Fulton and 1961 by W. F. Loomis. 



