238 PATTERNS AND PROBLEMS OF DEVELOPMENT 



However, decision between proctodeum and stomodeum is neither pos- 

 sible nor necessary. The neck is apparently a secondary ectodermal modi- 

 fication resembling an everted proctodeum or stomodeum. 



In partly entodermized forms there is also entodermal reconstitution; 

 entodermization is itself a reconstitution. The original prospective ento- 

 derm may form only a small part of the total entoderm, much less than 

 under natural conditions, and probably in some cases it is completely dis- 

 sociated; but development, either of entodermized region together with 

 original entoderm, or of the former alone, is definite and orderly and ap- 

 parently entodermal in character. Three entodermal regions, separated 

 by constrictions, often develop secondarily; or after greater inhibition, 

 only two, sometimes with a very small third region at the tip; or with 

 still more inhibited development there may be no regional differentiation. 

 Supposedly, the three or two regions correspond to entodermal regions 

 differentiating in normal development; but if this is so, how is the regional 

 differentiation determined in an external entoderm consisting in part of 

 original entoderm, in part of entodermized ectoderm, or entirely of the 

 latter? From the data available, it appears that regional differentiation 

 of entodermized ectoderm does not occur unless some ectoderm remains, 

 and it may be absent or almost absent if the ectoderm is very small 

 (Fig. gi, H); but whether absence of entodermal differentiation indicates 

 absence of some determining or inducing action of ectoderm, or is merely 

 the result of extreme inhibition, is at present not known. In any case, 

 differentiation of entodermized ectoderm with or without original ento- 

 derm and without invagination suggests orderly and definite interrela- 

 tions between parts, even in the differentially modified forms. 



Entoderm which has lost its epithehal character during exposure to 

 lithium or other agents may, if exposure is not too long, regain epithelial 

 order to som^e extent during recovery in water. For example, as noted 

 above, forms like A and B of Figure 94 may become exogastrulae like 

 E-H of Figure 91. That there is recovery of epithelial character in these 

 cases seems evident. Moreover, after extreme inhibition spherical ento- 

 dermal vesicles often appear free in the blastocoel during recovery; ex- 

 amples are shown in Figure 96. Individuals in which the whole basal 

 region has become merely a cell mass with more or less dissociation (Fig. 

 96, A) may, after return to water, attain the condition of Figure 96, B, 

 with large external epithelial sac, many dissociated cells in the blastocoel, 

 and one or more epithelial vesicles. Many individuals like Figure 96, C, 

 do not recover, but 20-25 per cent may attain the condition of Figure 96, 



