404 C. M. CHILD. 



a susceptibility gradient and that indications of susceptibility 

 differences in the minor axes have been observed. 



The apico-basal susceptibility gradient is the same as that in 

 the starfish (Child, '150) and in all other forms examined, at 

 least in early developmental stages and in many cases throughout 

 life. That this gradient is of fundamental significance cannot 

 be doubted and I have attempted elsewhere (Child, 150) to 

 present some of the evidence which seems to me to indicate that 

 a physiological axis is fundamentally such a gradient in metabolic 

 rate. 



The chief points are summarized as follows: 



1. A distinct gradient in susceptibility to potassium cyanide, 

 ethyl alcohol, ammonium hydrate and hydrochloric acid is present 

 along the apico-basal axis of blastula, gastrula, and later stages 

 of larval development of Arbacia and indications of a gradient 

 are found in earlier stages. 



2. In the apico-basal gradient the susceptibility is highest at 

 the apical end of the axis and lowest at the basal end. Since 

 susceptibility to these reagents varies in general directly with 

 metabolic rate, the susceptibility gradient indicates the existence 

 of a gradient in rate of metabolic activity in which the rate is 

 highest in the apical region and decreases basally. 



3. Some indications of gradients in other axes appear in the 

 differences of susceptibility, but these are much less distinct 

 than the differences along the apico-basal axis. 



4. The anal arms and the posterior end of the larval body ap- 

 pear as secondary regions of high susceptibility after they begin 

 to develop. 



5. In the fully developed pluteus these susceptibility gradients 

 become less marked and doubtless disappear as metamorphosis 

 begins. 



HULL ZOOLOGICAL LABORATORY, 

 UNIVERSITY OF CHICAGO, 

 February, 1916. 



