DIFFERENTIAL DEVELOPMENTAL MODIFICATION. II 233 



suits from a reversal of cell polarity in the prospective entoderm, there is 

 no evidence of such reversal in entodermized ectoderm. It remains in 

 essentially the same physical relations to other parts as when it was 

 prospective ectoderm of the blastula. So-called "exogastrulae" of this 

 type are actually not exogastrulae at all but partly entodermized blastulae 

 with more or less regional differentiation. For convenience and in a purely 

 descriptive sense they may be called "exogastrulae" because their ento- 

 derms are external instead of internal, but it should not be forgotten that, 

 as regards origin and development, they are very different from true 

 exogastrulae; in so far as the entodermized ectoderm is concerned, they 

 are pseudo-exogastrulae. Entodermization of ectoderm apparently has no 

 relation to gastrulation. 



Since the physical relations of entodermized ectoderm to other parts 

 are not altered by its entodermization and there is no evidence of reversal 

 of polarity in its cells, it remains a question whether the external entoderm 

 formed by it is inside out and which end is physiologically its apical end. 

 The end attached to the remaining ectoderm has developed from the 

 higher, more nearly apical, level of the prospective ectoderm. Moreover, 

 differential dye reduction indicates that this end usually shows higher 

 rate of dye reduction.'^ 



EXOGASTRULATION AS A DIFFERENTIAL MODIFICATION OF DEVELOPMENT 



In general, the modifications of development by lithium do not differ 

 essentially from those produced by other inhibiting agents. All degrees 

 of inhibition of ectodermal development appear with lithium, as with 

 other agents; and the secondary modifications resulting from differential 

 tolerance, conditioning, or recovery are the same in character with lithium 

 and other agents. Even entodermization of prospective ectoderm is not a 

 specific effect of lithium but is produced by various other inhibiting 

 agents — -crowding and Janus green in Dendraster, several agents in Patiria; 

 further investigation will probably show that many other agents have a 

 similar effect. In fact, the evidence suggests that entodermization may 

 be a nonspecific differential inhibition. Differential susceptibility to grad- 

 ually lethal action of many agents and to inhibiting action on develop- 

 ment and differences in rate of dye reduction in low oxygen's indicate a 

 single gradient of physiological condition in earlier pregastrular stages of 

 echinoids and asteroids, with decrease in susceptibility and rate of dye 



'^ Fig. 95, A,B,F, G, H, and Child, 19366. 



'5 Chap, iv and preceding sections of present chapter. 



