DIFFERENTIAL DEVELOPMENTAL MODIFICATION. II 207 



of mesenchyme at the time of immigration is more or less inhibited; and 

 before tolerance, conditioning, or recovery occurs, it is too late for normal 

 skeletal development. These forms also are not specific for any particular 

 agent, but their appearance in a given species depends on concentration 

 or intensity, developmental stage, and period of exposure. Although the 

 skeleton may consist of a single rod transversely oriented, these forms are 

 of quite different origin from those of Figure 75, E-H, which are results 



Fig. 79, A-G. — Secondary modifications in Arbacia. A, B, lateral and basal outlines, con- 

 tinuous in alcohol, 1.5 per cent, with apical region secondarily modified, basal region differ- 

 entially inhibited; C, D, lateral and basal outlines, CUSO4 m/2, 500,000, continuous; E-G, 

 secondary modifications in apical region only, ventrodorsality obliterated, CuSO^ m/2, 500,000, 

 continuous. 



of relatively extreme inhibition with almost complete or complete oblitera- 

 tion of ventrodorsality and decrease of polarity. 



The most extreme secondary modifications resulting from differential 

 conditioning with continuous exposure to low concentrations of agent have 

 been obtained with ethyl alcohol and CUSO4. In Figure 79, A and B 

 (alcohol, 1.5 per cent) and C and D (CUSO4 m/2, 500,000) are examples. 

 In these the elongation and enlargement of the apical region is secondary, 

 early development being inhibited. Inhibition persists in the basal region, 

 arms being completely absent and the skeletal rods oriented at less than 

 the usual angle {B), or arms are present and greatly inhibited, but the 

 angle of the skeletal rods is slightly wider than normal, suggesting slight 



