DIFFERENTIAL INHIBITION IN OBELIA 489 



which cannot be mistaken. There is then no more resorption 

 and the cells macerate, and later disintegrate, without dedif- 

 ferentiation. 



Kesorption is a result of the process of migration and it 

 could not take place were the elements to be resorbed to remain 

 in their differentiated condition. Resorption then is consequent 

 on dedifferentiation. It occurs in many forms wdien a cavity- 

 is present into which the migration may occur (Child, 1904 ; 

 Huxley, 1921 &). 



In Obelia, as in Perophora and probably in many other 

 cases, if the cavitj^ into which migration can occur be by some 

 method or other limited, dedifferentiation with no or slight 

 resorption may take place (Text-fig. 6, p. 485). 



It is then dedifferentiation plus resorption that Loeb 

 means by ' liquefaction '. But this is not a specific result of 

 contact with hard surfaces. Whether such contact by itself 

 can produce the effect we do not know, but as an unfavourable 

 condition it can and does accelerate it. Contact stimulates 

 tentacles to contract, and constant stimulation must be 

 unfavourable ; oxidation must also be reduced in proximity 

 to the substratum. Loeb's analogy between the liquefaction 

 of the zooid and the clotting of blood, both due to contact with 

 solid objects, thus cannot stand. 



It may be said that dedifferentiation implies potential sub- 

 sequent redifferentiation. There is, however, no reason why 

 dedifferentiation should be reversible any more than differen- 

 tiation. If we lay down that dedifferentiation is reversion to 

 a morphologically simpler state with lower energy -requirements, 

 the simpler condition being preserved for a considerable time 

 and not merely a stage in the process of dying, we have 

 a good working definition. 



Dedifferentiation is the accepted term to denote the simplifi- 

 catory processes undergone by differentiated cells in tissue 

 culture ; and in this case there is usually no redifferentiation, 

 the tissues merely remaining alive for a longer or shorter time 

 in their simplified condition. Smooth muscle grown in culture 

 solutions dedifferentiates to a condition in which the cells divide 



