LARVAL DEVELOPMENT IN THE SEA URCHIN 71 



In short, these experimental methods produce two opposed 

 types of teratological forms with various gradations between 

 them, and the production of the two types can be controlled to a 

 very considerable degree, the differences in susceptibility in dif- 

 ferent individual eggs and different lots being the chief limiting 

 factors in control. For convenience the forms produced are 

 described under the four heads: direct differential inhibition, 

 differential acclimation, differential recovery; differential in- 

 hibition with general recovery; and consideration of the ques- 

 tion of control follows the description. The figures of the tera- 

 tological forms are semidiagrammatic and are intended to show 

 the form, axial relations, and chief structural features, details 

 of mesenchymal distribution and slight skeletal variations being 

 usually omitted. Wherever the distinction has seemed neces- 

 sary or desirable, the middle region of the enteron has been 

 drawn with a double contour, as its walls are thicker than those 

 of the oesophageal and rectal regions which are indicated by 

 single contours. All figures were drawn directly from living 

 material. 



THE FORMS RESULTING FROM DIRECT DIFFERENTIAL INHIBITION 



The results of direct differential inhibition are most clearly 

 seen where the development takes place in KCN, for little or 

 no acclimation to KCN occurs within the short period of devel- 

 opment from egg to larva. In NH 4 OH, development also occurs 

 without appreciable acclimation. Some degree of acclimation 

 to NaOH occurs during development, but in alcohol and acids 

 acclimation occurs so much more rapidly and completely than 

 in the other agents used, that concentrations high enough to 

 produce a large percentage of partial or total death must be 

 employed to produce the forms characteristic of differential 

 inhibition. 



Considering in order the various degrees of departure from 

 the normal form, the first appreciable differential inhibition 

 appears as a slight change in the proportion of parts in the 

 pluteus. A comparison of figures 6 A and 6 B, drawn from a 

 very slightly inhibited pluteus, with figures 5 A and 5 B, (a 



