LARVAL DEVELOPMENT IN THE SEA URCHIN 115 



eration of antero-posterior and medio-lateral differences and 

 finally of apico-basal differences. 



8. The changes in differential acclimation and differential re- 

 covery are in the opposite direction. They consist in increase 

 in size and over-development of the oral lobe, increase to 180° in 

 the angle of divergence between the arms, and over-develop- 

 ment of anterior and median as compared with posterior and 

 lateral regions. 



9. Where the effect of differential inhibition persists after gen- 

 eral recovery, a great over-development of the skeleton may 

 occur, since the mesenchyme cells are relatively less inhibited 

 than most, if not all other parts. 



10. Forms produced by the more extreme degrees of differen- 

 tial inhibition resemble, in certain respects the forms of the 

 more primitive echinoderm groups, and the wide-angled plutei 

 resulting from differential acclimation resemble larvae of ophiu- 

 rids. These facts suggest that in the evolution of the pluteus 

 larva from the primitive larval form, changes in the metabolic 

 relations along the axial metabolic gradients and perhaps the 

 establishment of conditions which determine new gradients, have 

 played an important part. 



11. All the experimental data indicate that the spatial orders 

 in the larval development of the sea urchin are fundamentally 

 gradients in rate of general metabolic reaction, with highest rates 

 of reaction determining apical, anterior and median regions. 



12. The different agents used differ in degree in their inhib- 

 iting action, but there is no evidence in the changes of form pro- 

 duced of any specificity of action. The order of decreasing 

 effectiveness in inhibiting development is as follows: cyanide, 

 ammonium hydrate, sodium hydrate, ethyl alcohol, acids. This 

 order is also the order of increasing capacity of the sea urchin 

 to become acclimated to, or to recover from the effects of these 

 agents. 



13. The relation between susceptibility and metabolic rate 

 affords a method for analytic teratogenic investigation and a 

 basis for the interpretation of many cases of experimental tera- 

 togeny already recorded and also of many teratological forms 

 observed in nature. 



