198 C. M. CHILD. 



SUMMARY. 



1. Differential inhibition of planula development can be 

 brought about in the hydrozoan, Phinlidinm gregarium, by 

 KCN, LiCl, ethyl urethane HC1 (probably CO 2 ) and by the 

 presence of the medusa? (CO 2 ). Resulting modifications range 

 from slight increase and extension of basal thickening of blastula 

 wall followed by development of slightly modified planula? to 

 spherical, solid, apparently apolar forms. In these immigration 

 and mass ingrowth are increased and take place from all parts 

 of the wall, and further development occurs only if a new polarity 

 arises. 



2. In differential acclimation in low concentrations of inhibiting 

 agents and differential recovery after return to water further 

 development of persisting axes may occur or new polarities may 

 arise through differential exposure of the surfaces of apolar 

 forms or through "adventitious" localization of regions of 

 greater activity. In rapid acclimation or recovery such axes may 

 develop directly as hydranth-stem axes, but usually they are at 

 first still more or less inhibited and develop as stolons and their 

 tips transform later into hydranth-stem axes, or such axes arise by 

 budding from their free surfaces. 



3. The stolon represents a polarity, a gradient, at a lower level 

 of physiological activity than the hydranth-stem axis. Transfor- 

 mation of hydranth-stem into stolon results from inhibition, of 

 stolon into hydranth-stem from acceleration; neither involves 

 necessarily a reversal of polarity. 



REFERENCES. 



Child, C. M. 



'15 Individuality in Organisms. Chicago. 



'23 The Axial Gradients in Hydrozoa. V. BIOL. BULL.. XLV. 



'24 Physiological Foundations of Behavior. New York. 



'25 The Axial Gradients in Hydrozoa. VI. BIOL. BULL., XLVIII. 



Lund, E. J. 



'21 Experimental Control of Organic Polarity by the Electric Current. I. 



Jour. Exp. Zool.. XXXIV. 

 MacArthur, J. W. 



'24 An Experimental Study and a Physiological Interpretation of Exogastrula- 

 tion and Related Modifications in Echinodenn Embryos. BIOL. BULL., 

 XLVI. 



