LARVAL DEVELOPMENT IN THE SEA URCHIN 85 



Where inhibition is slight and limited to the earlier stages of 

 development, differential recovery may appear as elongation of 

 the apical region in the gastrula (figs. 54, 55) or in prepluteus 

 stages (fig. 56). Such forms give rise to plutei like the acclima- 

 tion plutei of figures 23 and 24 with very large and long oral 

 lobes, wide angles of divergence, and short body. 



From forms of this sort, the gradation, through various com- 

 binations of differential inhibition and differential recovery, to 

 forms which show no differential recovery is complete. Cer- 

 tain features of these forms require some consideration. 



Where the inhibition is so great that minor gradients are 

 almost or quite obliterated, while the apico-basal gradient still 

 remains an effective factor in development, recovery produces a 

 series of forms resembling the differential acclimation forms of 

 figures 37 and 40 to 53. Figures 57 to 73 show examples of 

 differential recovery after a considerable degree of inhibition 

 by cyanide. In all cases the minor axes are almost or quite 

 obliterated, but there is usually elongation in the apico-basal 

 axis, and in many cases (figs. 57 to 64) an apical outgrowth, a 

 rudimentary oral lobe, on which the apical portion of the ciliated 

 band differentiates. Frequently also the basal portion of the 

 ciliated band differentiates as a more or less complete basal ring 

 (figs. 57 to 62, 65 to 72) as in figure 37. Here, as in differential 

 acclimation (p. 79), the differentiation of the basal portion of the 

 ciliated band as a partial or complete basal ring is associated 

 with the almost complete obliteration of the antero-posterior 

 axis and of bilaterality by the differential inhibition. This 

 basal ring appears much more frequently in recovery than in 

 acclimation, because, after a given degree of inhibition, differ- 

 entiation proceeds somewhat further when the animals are re- 

 turned to water than in the continued presence of the inhibiting 

 agent. The development of the apical outgrowth and the apico- 

 basal elongation are features of differential recovery, but the 

 basal ciliated ring is a consequence of general recovery (or accli- 

 mation, fig. 37) following a degree of differential inhibition which 

 almost or quite obliterates the minor axes. Both apical out- 

 growth and basal ciliated ring may be present (figs. 57 to 62) 



