LARVAL DEVELOPMENT IN THE SEA URCHIN 91 



Such larvae as these show the extremes of modification in the 

 direction opposite to the differential acclimations in figures 29 

 to 36, and the extremes in the two groups represent the limits 

 of possibility. In the one case, figures 82 to 87, parts, normally 

 lateral, approach the median line, while in the other, figures 29 

 to 36, parts normally longitudinal become transverse. And yet 

 these extreme differences in form are brought about by altering 

 the metabolic differentials along the axial gradients, in the one 

 case by decreasing the differences in metabolic rate along the 

 axes through differential inhibition, in the other by increasing 

 these differences through differential acclimation. 



One other interesting feature appears very frequently in cases 

 of recovery, particularly after KCN, where the inhibition has not 

 been extreme, but the differential effect is not fully compen- 

 sated by differential recovery. This is an extreme development 

 of skeletal structure as indicated in figures 88 to 90. The skele- 

 ton of the anal arms in such cases is composed of many partially 

 fused rods, the basal part of the body-skeleton and the posterior 

 spiny enlargements may be of enormous size (figs. 88, 89), and 

 additional short rods may arise from the regions of junction of 

 the different rods, and even at the median junction of the two 

 transverse basal rods, as in figure 89. Figure 90 shows a mul- 

 tiplication of arm rods in a more inhibited form. No attempt 

 has been made to figure the more extreme cases of this skeletal 

 over-development. In some the body seems to be largely filled 

 with skeletal structures, and very aberrant shapes often result 

 from the outgrowth of rods in various directions. In such cases 

 skeleton development seems to have run wild. This skeletal 

 over-development is undoubtedly a result of differential inhi- 

 bition, but of course occurs only in cases where general recovery is 

 brought about by return to water after temporary inhibition. 



It has already been pointed out that the susceptibility of the 

 mesenchyme cells is relatively low, as their origin from the 

 basal region of the embryo might lead us to expect. This being 

 the case, they are less affected by slight temporary inhibition 

 than are the more susceptible parts, and therefore, when the 

 metabolic rates of all parts are raised on return to water, the 



