LARVAL DEVELOPMENT IN THE SEA URCHIN 79 



gree of over-development, as compared with the norm, is in the 

 regions of highest rate, i.e., apical and anterior, and apparently 

 also median; and the greatest degree of under-development is 

 basal, posterior and apparently lateral. In consequence of 

 these changes in relative metabolic rate along the axes, the larva 

 is transformed from the antero-posteriorly elongated normal 

 form (figure 3 A) into a broad, transversely flattened, form with 

 short antero-posterior axis ; and all stages of this transformation 

 appear. 



In cases where the degree of inhibition is somewhat greater, 

 differential acclimation in acid gives rise to forms like figure 37, 

 which shows basal (A), lateral (B), and anterior (C), aspects 

 of a characteristic acid form. In this larva the ciliated band, 

 which, in the normal larva and the less modified forms, extends 

 as a continuous band around the margins of the anterior end 

 and over the oral lobe and basal arms, is differentiated only in 

 the apical and basal regions, as indicated in figure 37 by the 

 shaded bands. The apical portion of the band extends around 

 the rudimentary oral lobe and a short distance basally, while the 

 basal portion extends from the medial anterior region posteriorly 

 more than half way around the body, and the lateral portions, 

 which normally connect these apical and basal portions, are not 

 present. In fact, the basal portion of the ciliated band in such 

 cases forms a more or less complete ring around the basal region. 

 This condition recalls the condition in the more primitive type 

 of echinoderm larvae where several ciliated bands surround the 

 body. Apparently in such cases as this, where the antero- 

 posterior axis and bilaterality are practically obliterated, the 

 local metabolic conditions determine that the ciliated band shall 

 develop around the basal region, instead of over the arms and 

 around the lateral margins of the anterior region. This con- 

 dition also appears frequently in recovery forms (pp. 85, 87, 88). 



When the direct inhibiting action of the agent is somewhat 

 more lasting, and acclimation occurs somewhat more slowly, as 

 in alcohol, the changes resulting from differential acclimation are, 

 so to speak, superimposed on the changes resulting from dif- 

 ferential inhibition. In such cases, therefore, various combina- 



