LARVAL DEVELOPMENT IN THE SEA URCHIN 87 



its normal relations, or rupture may occur between stomach- 

 intestine and rectal region, the latter being left in connection 

 with the blastopore and usually degenerating. The exact his- 

 tory of each particular case can be determined only by isolation 

 and continuous observation of individuals, and this I have not 

 attempted. 



Although no skeleton develops in forms of the character of 

 figures 57 to 73, there is in nearly every case, as in similar accli- 

 mation forms, a distinct aggregation of mesenchyme cells in the 

 basal region where the skeleton normally arises, and the condi- 

 tions which determine it are evidently the same as in acclima- 

 tion (p. 81). 



It is probable that at least many of the forms in figures 57 

 to 73 have lost some part of the apical ectoderm by apical par- 

 tial death, resulting from the direct differential action of the 

 agent, and that the apical outgrowth is therefore,, in the strict 

 sense, a reconstitution of the apical region. It is certain that 

 such forms may develop, both where a part of the apical region 

 has died and where there have been no apical losses by death, 

 so that the question, whether the apical outgrowth is a reconstitu- 

 tion of the apical end from cells which were originally not apical, 

 has little significance. Physiologically, it is a reconstitution, 

 whether apical losses have occurred or not, for with a sufficient 

 degree of inhibition the apical cells themselves are incapable of 

 developing an oral lobe, but when their metabolic rate rises to a 

 certain level, in consequence of differential acclimation or re- 

 covery, they become capable of such development, and, since 

 they acclimate or recover more rapidly than cells below them, 

 the development may take the form of a local reconstitutional 

 outgrowth. 



These cases of recovery are of the same general type as the 

 cases of differential acclimation in figures 37, and 40 to 46. In 

 both groups the antero-posterior axes and bilaterality are al- 

 most or quite obliterated, and the apico-basal axis remains as 

 the chief or only effective factor in orderly development. 



As noted above (p. 79) forms of this kind recall, in various 

 ways, the more primitive types of echinoderm larvae. In the 



