92 C. M. CHILD 



metabolic rate of the mesenchyme cells is relatively higher, 

 as compared with other parts, than in uninhibited animals. Be- 

 cause of this difference, they are able to obtain more nutrition 

 from other parts and so to undergo more growth and division 

 and, finally, to give rise to a larger amount of skeletal substance 

 than normally. The normal metabolic relations between the 

 mesenchyme and other parts have been altered to the advantage 

 of the mesenchyme cells by the differential inhibition. Since 

 the mesenchyme rises from the basal region of the egg, this effect 

 of differential inhibition upon the mesenchyme is merely a 

 special case of differential inhibition along the apico basal gra- 

 dient, the mesenchyme cells (or the region from which they arise), 

 being less inhibited because of lower metabolic rate. Since the 

 mesenchyme cells do not remain a part of the apico basal axis, 

 but are distributed in certain relations to other parts, the effect 

 of differential inhibition in this case is simply an over-develop- 

 ment of the skeleton to which they give rise. 



THE CONTROL OF THE DIFFERENTIAL MODIFICATIONS 



Eliminating, as far as possible, changes of temperature, con- 

 stitution of sea water and other variable conditions, the control 

 of the results with any one of the agents used is a matter of 

 concentration, stage of development at which the inhibiting 

 action begins, and, in the case of temporary action of the agent, 

 the length of the time of action. Where the agent begins to act 

 at a sufficiently early stage of development, higher concentra- 

 tions produce differential and general inhibition; lower concen- 

 trations, differential acclimation, and the shorter the period of 

 action the higher the concentration required to produce a par- 

 ticular modification. In experiments on differential inhibition 

 and differential acclimation, all six agents, — KCN, C 2 H 5 OH, 

 HC1, CILCOOH, NH4OH, and NaOH— were used, but in the 

 study of differential recovery after temporary action, only KCN 

 and alcohol were employed, for the close of the breeding season 

 of Arbacia terminated the work before the data could be 

 completed. 



