DIFFERENTIAL INHIBITION IN OBELIA 



479 



Table III (Obelia). 



Experiments in the N/256,000 solution were repeated several 

 times with the same results. These results indicate that a 

 N/256,000 solution of KCN inhibits the zooids without affecting 

 the hydrocaulus to any appreciable extent (at least for a con- 

 siderable time — fifty hours). The healthy nature of the 

 hydrocaulus is evidenced by movements of contraction and 

 pulsation, and by growth at the proximal extremity. 



The contraction of the stem is of interest, since precisely 

 similar contraction occurs in Perophora and other Ascidians 

 (Huxley, 19216). In addition, the partly dedifferentiated 

 zooid also appears to contract at intervals (Text-fig. 3), although 

 it is possible that the contraction is a mere surface-tension 

 effect, exerted passively on relaxation of the walls of the stem. 

 It appears that the contraction of the cells of the stem occurs 

 when considerable internal tension has been produced through 

 the flow of liquid and cells from the zooid. In higher forms 

 embryonic cells which are destined to give rise to muscle appear 

 to start contracting before differentiation, also as a result of 

 tension (e.g. Carey, 1921 a and 1921 h, &c.). 



It is possible that contraction produced by tension has no 

 normal function in hydroid stems. It is all the more interesting 

 to find that it occurs, being thus probably a general property 

 of all not too highly-differentiated cells. Possibly tension acts 

 also as a stimulus to outgrowth from the stem. (Cf. the well- 

 known fact of better growth in regenerating Tubularia in 

 hypotonic sea-water (Loeb, 1892).) 



