CONTENTS 



Crozier, W. J., and Arey, L. B. The heliotropism of Onchidium: 

 a problem in the analysis of animal conduct 107 



Northrop, John H. The combination of enzyme and substrate. 



I. A method for the quantitative determination of pepsin. 



II. The effect of the hydrogen ion concentration 113 



Maxwell, S. S. Labyrinth and equilibrium. I. A comparison of 

 the effect of removal of the otoHth organs and of the semicir- 

 cular canals 123 



Harvey, E. Newton. Studies on bioluminescence. X. Carbon 



dioxide production during luminescence of Cypridina luciferin. . 133 



Harvey, E. Newton. Studies on bioluminescence. XI. Heat pro- 

 duction during luminescence of Cypridina luciferin 137 



CoHN, Edwin J., Gross, Joseph, and Johnson, Omer C. The 



isoelectric points of the proteins in certain vegetable juices 145 



Swingle, W. W. Iodine and the thyroid. IV. Quantitative experi- 

 ments on iodine feeding and metamorphosis 161 



Loeb, Jacques. Influence of the concentration of electrolytes on 

 the electrification and the rate of diffusion of water through 

 collodion membranes 173 



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