LARVAL DEVELOPMENT IN THE SEA URCHIN 77 



occur, but in alcohol and hydrochloric and acetic acids acclima- 

 tion occurs much more rapidly and to a much greater degree, 

 so that differential acclimation is often evident in the early gas- 

 trula stage. Consequently, with these agents much more ex- 

 treme modifications of form can be produced through acclima- 

 tion. NaOH is intermediate between these two groups as 

 regards the forms resulting from differential acclimation. 



The earliest marked indications of differential acclimation 

 which have been observed occur in alcohol and acids, and consist 

 in increase in the relative size and growth of the apical region 

 of the gastrula. In acids this is very commonly a tapering pro- 

 longation of this region (fig. 25), while in alcohol it is more often 

 rounded (fig. 26 B), but both forms may appear in both agents. 

 Irregularity of outline and roughness of the external surface of 

 the basal region of the gastrula (figs. 26 .A, B), is particularly 

 characteristic of the action of alcohol, though often produced to a 

 lesser degree by the acids. It is evidently due to a decrease in 

 the normal epithelial coherence of the cells in this region, but' as 

 development and acclimation proceed, it almost or quite dis- 

 appears. 



Where the apical region undergoes a relative acceleration in 

 stages as early as the gastrula, disproportion between it and 

 other parts increases as development proceeds. Figures 27 and 

 28, are side views of prepluteus stages showing this dispropor- 

 tion. As compared with the normal form at this stage (fig. 3 B) 

 the apical and anterior regions are relatively greatly over-devel- 

 oped, and basal and posterior regions, including skeleton and 

 arms are greatly under-developed. These alterations are exactly 

 the opposite of those resulting from differential inhibition (fig. 

 7 B). In the acids, where acclimation occurs more rapidly and 

 the differential features are more marked than in any other agents 

 used, the most extreme modifications of this sort occur. Figures 

 29 and 30, A basal, B lateral aspect, show cases from acid in 

 which there is enormous over-development of apical, as com- 

 pared with basal regions, and in which the anterior region is so 

 much over-developed that the arms approach or attain an angle 

 of 180°, while in more posterior regions the angle of divergence, 



