LARVAL DEVELOPMENT IN THE SEA URCHIN 81 



forms show differential acclimation in the more apical regions 

 and differential inhibition in the more basal. 



Figures 40 to 46 show the final stages of differential acclima- 

 tion in alcohol after still greater degrees of inhibition. In all 

 figures the apical end is uppermost, but the antero-posterior axis 

 has been obliterated to such an extent that in most cases it is 

 impossible to distinguish anterior and posterior regions. The 

 most apical portion of the ciliated band is differentiated in figures 

 40 to 43, but in figures 44 to 46 differentiation ceases at an earlier 

 stage. The different positions, degrees of differentiation, and 

 relations of the entoderm depend upon the relations between the 

 time when apical acclimation began, upon the degree of dif- 

 ferentiation of the entoderm and upon the degree of direct 

 inhibition. Where the direct inhibition does not bring about 

 closure of the blastopore (fig. 15), the entoderm may separate 

 between stomach-intestine and rectal regions, the latter re- 

 maining at the basal end and undergoing degeneration (fig. 40), 

 or elongating and maintaining its normal basal connection (fig. 

 41). In other cases the blastopore closes completely, and the 

 entoderm loses connection with the blastopore region, but re- 

 mains in contact with the apical region of the body wall, and 

 is carried with it when elongation or enlargement occur. Under 

 these conditions it may differentiate completely, or the rectal 

 region (fig. 40), or mouth and oesophagus (figs. 41, 42) may be 

 absent, or lastly, it may remain a rounded vesicle (figs. 43 to 46). 



Where the entoderm remains a closed vesicle it often undergoes 

 degeneration after a few days (figs. 47, 48) as in differential 

 inhibition (p. 74), leaving anenteric forms which may remain 

 alive for several days longer. 



Even though no skeleton develops in these forms, more or less 

 aggregation of mesenchyme in the basal region occurs as in 

 normal development, while, in the more extreme degrees of in- 

 hibition without acclimation, the mesenchyme remains irregu- 

 larly scattered in the blastocoel. Evidently in acclimation the 

 apico-basal metabolic differences are sufficient to constitute effec- 

 tive factors in determining the localization of the mesenchyme, 

 while a sufficient degree of inhibition makes them ineffective. 



JOURNAL OF MORPHOLOGY, VOL. 28, NO. 1 



