402 C. M. CHILD. 



The susceptibility gradient in the earlier stages of this trans- 

 formation is the same as in the gastrula, apico-basal (Fig. 19) 

 and the same difference in susceptibility between ectoderm and 

 entoderm persists. Later, when the larva begins to elongate in 



20 



the antero-posterior direction, and the anal arms begin to develop, 

 these arms and the posterior end both appear as secondary 

 regions of high susceptibility (Fig. 20) though the susceptibility 

 of the anal arms is in general somewhat less than that of the oral 

 lobe and that of the posterior end somewhat less than that of the 

 anal arms. On the oral lobe and over the body death progresses 

 in the basal and posterior direction and in the anal arms from 

 tip to base of the arms. 



In the fully developed pluteus the susceptibility gradients are 

 less marked. The ectoderm of the oral lobe and anal arms is still 

 somewhat more susceptible than that of other regions but the 

 differences are less conspicuous. In all these later stages, how- 

 ever, the entoderm remains much less susceptible than the ecto- 

 derm and apparently the mesenchyme is least susceptible of all 

 parts. 



It is probable that the gradual fading out of the metabolic 

 gradients in the pluteus is a physiological change which precedes 

 and makes possible the development of the axial gradients of the 

 mature sea urchin which have been previously inhibited by the 

 existing axial relations. If this suggestion is correct these changes 

 are the factors which determine, or rather permit metamorphosis. 



The development of the arms and the posterior elongation 

 has been shown to be dependent on the development of the 

 skeleton. That being the case the appearance of high suscepti- 

 bility in the anal arms and the posterior end is not self-determined 

 in these parts but results from skeletal growth. 



