DIFFERENTIAL DEVELOPMENTAL MODIFICATION. II 221 



tion, that is, the apicodorsal region is so strongly inhibited that only on 

 the less inhibited ventral side is coelom development possible. Oblitera- 

 tion and reversal of the normal coelomic asymmetry also represent dif- 

 ferential modifications and are characteristic results of certain degrees of 

 inhibition by all agents used. 



It is possible that with sufficient inhibition of the more susceptible 

 region the normally less susceptible and presumably less active region is, 

 to some extent, physiologically isolated and undergoes activation, par- 

 ticularly after return to water. If the left coelom normally represents a 

 higher level of activity and is in some degree dominant, differential in- 

 hibition may permit sufficient activation of the right side to reverse asym- 

 metry. Dorsiventral differential inhibition of the foregut may bring about 

 physiological isolation and permit activation of the ventral side and de- 

 velopment of a coelom there after return to water. But whether physio- 

 logical isolation is or is not involved, it seems evident that localization of 

 coelom development on the foregut is determined by nonspecific factors, 

 quantitative differences in physiological condition, rather than by defi- 

 nitely localized, "coelom-forming substances." With recovery after rela- 

 tively extreme inhibition of the apical entodermal region three or even 

 four coelum buds may develop, usually equidistant from each other, 

 about the circumference of the radial or almost radial foregut. In these 

 cases asymmetry has been obliterated, there is little or no evidence of 

 ventrodorsahty, and the whole circumference of the foregut is appar- 

 ently equipotent for coelom. The question how the coelom buds are 

 localized about the circumference of the foregut in a particular case is 

 the same as that concerning locahzation of individual tentacles about the 

 circumference of the body in various hydroids. The small coelomic ves- 

 icles developing from the posterior region of the midgut also show change 

 in position, number, and asymmetry under inhibiting conditions and with 

 secondary modifications. In recoveries as many as six of them may de- 

 velop in a circle about the midgut. 



ECHINODERM EXOGASTRULATION AND ASSOCIATED MODIFICATIONS 



It was discovered by Herbst (1892, 1895, 1896a) that when early stages 

 of sea-urchin development were treated for a time, beginning soon after 

 fertilization or in early cleavage, with lithium (usually LiCl) in certain 

 concentrations the entodermal region evaginated at the stage of gastrula- 

 tion instead of invaginating. Another effect of lithium was differential 

 entoclermization of prospective ectoderm, progressing from basal ecto- 



