234 PATTERNS AND PROBLEMS OF DEVELOPMENT 



reduction basipetally from the apical pole and a change in condition, ap- 

 parently an activation, in the basal region- — in echinoids as gastrulation 

 approaches, in asteroids somewhat later. Prospective entoderm consti- 

 tutes the lower levels of the primary gradient. Exposure to lithium and 

 other agents entodermizes only when it begins in the earlier pregastrular 

 stages. In view of all the facts, it seems probable that the primary action 

 in entodermization is a depression or inhibition, rather than a direct in- 

 crease, in concentration and extent of a specific vegetal substance gradient 

 or stimulation of a specific vegetal metabolism, as Runnstrom maintains. 

 Lower levels of prospective ectoderm are entodermized by lesser degrees 

 of inhibition than higher levels because they are nearer the critical level. 

 This depression must result in alteration of concentration of many react- 

 ing substances in the cells concerned and in their relations to other parts, 

 and so in progressive increase of their specificity as entoderm. It has been 

 pointed out that lithium not only entodermizes but inhibits development 

 of original entoderm and of entodermized ectoderm. It is a point of some 

 importance that the exogastrulae with extremely large entoderms (Fig. 91, 

 B, C, E-G) are not direct effects of lithium or other agents but secondary 

 modifications, appearing in most extreme form in recovery but also to 

 some extent with differential tolerance or conditioning in low concentra- 

 tions. Comparison of Figure 90, A, E, and F, continuous exposures, with 

 Figure 90, B and C, recoveries after return to water, and of Figure 91, D, 

 continuous, with Figure 91, A-C and E-G, recoveries, will show the dif- 

 ference in entodermal development. 



In recovery after high concentrations of lithium, which practically stop 

 development in earlier pregastrular stages, secondary modifications are 

 often greater, if exposure is not too long, than after somewhat lower con- 

 centrations, apparently because with the higher concentrations, the stage 

 of activation of entoderm is reached only after return to water, while 

 with lower concentrations it may be reached and activation inhibited 

 during exposure. A comparison of Figure 91, A, C, and F, 7 hours in 

 LiCl m/25 from two-cell stage, with Figure 91, D, in m/50 from the same 

 stage, illustrates the point. 



Lithium, like other agents, inhibits more or less completely skeletal 

 development; but in secondary modifications, particularly in recovery 

 from the less extreme inhibitions, excess of skeleton with supernumerary 

 arms and other skeletal structures not normally present often develops. 

 Whether mesenchyme cells increase in number secondarily or dissociated 

 entodermal cells function as mesenchyme is not known. 



