DIFFERENTIAL DEVELOPMENTAL MODIFICATION. II 235 



The possibility that specific vegetal substance is already present in 

 early stages is, of course, not excluded; but the evidence seems to support 

 the view that the primary effect of lithium in entodermization, as in other 

 modifications, is depression or inhibition, an action on quantitative dif- 

 ferences in physiological condition, with origin or increase of specific dif- 

 ferences as a result. In short, according to this view, lithium and many 

 other agents are primarily inhibitors of early echinoderm development, 

 with regional differentials in effect depending on nonspecific differences 

 in physiological condition and their changes in the course of development. 

 Secondary differential modifications of tolerance, conditioning, or recov- 

 ery are not direct effects of the inhibiting agents but results of action of 

 the gradient factors of physiological pattern. 



There is considerable evidence that ectodermization of prospective en- 

 toderm and re-ectodermization of entodermized ectoderm are possible. 

 The forms of Strongylocentrotiis in Figure 92, D, E, and F, with exposure 

 to lithium beginning at late blastula stage, suggest partial ectodermiza- 

 tion, as does Figure 92, ^, of Dendraster. At the late blastula stage, or a 

 little later, entoderm becomes more susceptible than ectoderm; and it 

 seems not impossible that with depression or inhibition it may still be ecto- 

 dermized. The ectodermized and "animalized" forms discussed below (pp. 

 243-45) have a different origin. 



In exogastrulae with ectoderm decreased by entodermization the locali- 

 zation of mesenchyme near the apical ectodermal pole suggests that ento- 

 dermization extended earlier to, or almost to, that level but that with 

 secondary modification more or less re-ectodermization has left the mes- 

 enchyme where it was localized by the entodermization (Fig. 91, A-D). 

 Localization of mesenchyme at two or more levels may represent stages 

 in entodermization or in re-ectodermization. It was noted above that in- 

 dividuals with apparently completely entodermized ectoderm (Fig. 94, 

 A, B) may, in recovery, become exogastrulae with some ectoderm, like 

 Figure 91, E-H. These cases also indicate re-ectodermization. In a dif- 

 ferent line of experiment re-ectodermization has been observed by von 

 Ubisch (1925a, 1929). 



At present there seems to be no adequate reason for regarding either 

 true or pseudo-exogastrulae as primarily anything but results of differen- 

 tial inhibition, often with various degrees of secondary modification due 

 to differential tolerance, conditioning, or recovery. The question how any 

 agent brings about true exogastrulation remains. What determines evagi- 

 nation instead of invagination? If we knew how invagination is deter- 



