182 



C. M. CHILD. 



larva, usually without distinct boundary between ectoderm and 

 entoderm. Figure 10 shows an early stage, Figs. II and 12 show 

 later stages of these forms. 



10 



II 



13 



When exposure to inhibiting agents is begun only after the 

 blastula stage is attained, polar elongation, basal thickening and 

 immigration may occur in the agent. In the higher concentra- 

 tions, however, immigration or thickening of the blastula wall 

 gradually extends apically and may involve all regions of the 

 wall, the blastococl may be obliterated and the elongation may 

 disappear completely, the extreme modification short of death 

 being spherical solid, apparently completely apolar forms 

 indistinguishable from those described above (Figs. II, 12). In 

 these cases there is actual regression so far as visible axial dif- 

 ferences, elongation and basal thickening are concerned. 



These spherical solid larva*, whether they arise from earlier 

 stages before elongation occurs, or from later elongated stages by 

 regressive changes, are, so far as can be determined, apolar. 

 Susceptibility gradient and reduction gradient are no longer 

 present. They have lost the capacity. for definitely directed 

 locomotion and roll about on the bottom, being usually unable to 

 support themselves free in the water. They show no definite 

 attachment reaction, but gradually become quiescent and after 

 movement ceases adhere by some part of the surface and become 

 more or less flattened (Fig. 13). If conditions do not permit 

 acclimation or recovery, no further development occurs, though 

 they may live for two weeks or more. In short, the physiological 

 differences constituting polarity have apparently been obliterated 



