DIFFERENTIAL SUSCEPTIBILITY IN POLYCHETES 27 



stage), the result is a megacephalic form with more or less 

 inhibited posterior growing region. 



The results of differential inhibition of sea urchin development 

 (Child '16 c, as well as various other data as yet unpubhshed) 

 indicate that two factors are involved in the modifications of 

 development by this means. The first of these factors is the 

 direct effect of the agent upon each part. In general a decrease 

 in the metabolic activity of a developing, grooving part is ac- 

 companied by a decrease in the rate of growth and development. 

 The second factor is the alteration of the metabolic relaMons 

 existing between the various parts. The general proportions 

 and the relative sizes of the various organs in any organism must 

 be, to a greater or less extent, an expression of the relative metab- 

 olic activities of the different regions and parts. This is par- 

 ticularly true in the earlier stages of development, where metab- 

 olism is largely concerned with growth. If nutritive material 

 be present in excess, the region with the more intense metabolic 

 activity may simply grow more rapidly than a less active region, 

 until its growth is retarded by senescence or other conditions. 

 If nutritive material be limited and insufficient, a region of higher 

 metabolic rate may not merely grow more rapidly than, but 

 may live and grow at the expense of, a region of lower rate and 

 so may not only retard or prevent its growth, but may bring 

 about its reduction or atrophy. If we inhibit certain regions 

 of the annelid larva to a greater degree than others, we not only 

 retard this growth, but we make it possible for other less in- 

 hibited regions to grow relatively more rapidly- because nutritive 

 material which, under normal conditions, would go to supply 

 the demands of the more active regions, is thus made available 

 to a greater degree for other regions. The effect of this factor 

 will, of course, be in the same direction as the direct effect of 

 the differential inhibition of different regions and will merely 

 tend to increase the changes in form and proportion produced 

 directly by the differential inhibition. 



To what extent each of these two factors is concerned in the 

 modifications of form in the annelid larva it is impossible to 

 determine, but that they both play a part here as well as in 



