20 C. M. CHILD 



duce much differential effect, and HCl m/2000 usually kills in 

 a day or two. HCl m/2500 has been chiefly used for differential 

 inhibition. Higher concentrations of HCl with temporary 

 exposure have not as yet been used. 



Nereis 



Because of the impossibility of obtaining abundant material 

 as desired, the data concerning modification of development in 

 Nereis are rather fragmentary. Since the chief object of the 

 present paper is the demonstration of the general regional differ- 

 ences and changes in susceptibility and their relation to the form 

 and proportions of the larvae, the figures are drawn to illustrate 

 these points, most of the details of larval structure being omitted. 

 In all cases the figures show merely the dorsal aspect of the body, 

 because the side-views show in general the same modifications of 

 form and proportion as the dorsal. As regards the inhibition 

 or modification of particular organs and parts a few facts are 

 mentioned. There is no doubt that, with sufficient material, 

 much may be accomplished in the way of selective or differ- 

 ential inhibition of particular organs by varying concentrations 

 of agents used and periods of exposure over a wide range, and 

 beginning exposure at different stages of development. By 

 means of such a procedure a particular organ is most inhibited 

 when the period of action of the agent in high concentration 

 coincides with its period of highest susceptibility or highest 

 metabolic activity, and since this period occurs in different 

 organs at different stages of development, it is possible to inhibit 

 a particular organ relatively more at one stage, relatively less 

 at another, the selective action at a given stage being primarily 

 not a matter of specific chemical constitution but of metabolic 

 activity and susceptibility. My experiments also indicate that 

 something may be done in the way of differential acclimation 

 (Child '16 c) with low concentrations, at least with some agents, 

 although effects of this kind will appear only in the later stages, 

 so far as they appear at all. All these variations of the suscepti- 

 bility method are however limited in their applicability by the 

 rapid development and by the fact that the inhibiting agent 



