APPENDIXES 749 



nearly of exogastrulae usually results from i6 to i8 hours' exposure to m/so, m/40, or 

 m/30 from the two-cell stage. Lower percentages occur with lower concentrations, in- 

 creasing frequency of dissociation of the entodermal region, cessation of development 

 and death, with higher concentrations. The higher the concentration with a certain 

 exposure period the farther apically entodermization extends and the more frequent 

 is dissociation of the original prospective entoderm. Strongylocentrotus franciscanus 

 and S. purpuratus are apparently somewhat less susceptible to LiCl than Dcndraster, 

 and the first seems to be slightly less susceptible than the second. 



Patina is less susceptible than the echinoids. In m/30 for 14 hours or longer from 

 early cleavage only 5-10 per cent exogastrulae appear: in m/20 for 18-25 hours from 

 early cleavage 80-90 per cent are exogastrulae. With longer exposure to these concen- 

 trations more or less dissociation of the original prospective entoderm occurs, as in 

 echinoids. With exposure beginning in later stages the frequency of exogastrulation 

 and degree of entodermization decrease. Inhibition and more or less dissociation of 

 invaginating entoderm, particularly of the enlarging apical region, occurs in m/40 with 

 continuous exposure. 



With concentrations of LiCl which produce exogastrulation in Patina there is usually 

 little differentiation of ectoderm, but with lower concentrations the same differential 

 modifications of form and proportion appear as with other agents, primarily differ- 

 ential inhibition, secondarily the opposed modifications of differential tolerance, con- 

 ditioning, or recovery. Extensive entodermization of prospective ectoderm has been 

 obtained in Patina in Janus green 1/2,000,000. In the writer's experiments the sus- 

 ceptibility of Asterias forbesii was about the same as that of Patiria, but MacArthur 

 has reported exogastrulation in that species and in Orthasterias with LiCl m/ioo to 

 m/i6o. 



Differential inhibition from crowding probably results from lack of oxygen or from 

 some toxic metabolite rather than from CO2, since it may be extreme with pH no lower 

 than 7.3-7.4. Concentrations of the dyes used to produce differential modification have 

 little significance, provided they are low enough, for they are accumulated within the 

 cells even from extremely low concentrations in the water. 



