EMBRYONIC INDUCTORS AND ORGANIZERS 443 



constitution in hydroids and planarians. Isolation from higher levels of 

 the primary gradient results in activation of the more apical parts of 

 veg2 and raising of gradient-levels there, so that ectoderm, instead of 

 entoderm, develops; but the ectoderm does not attain full development, 

 probably because inhibited by the basal activation and gradient, much 

 as hydranth and planarian head are inhibited by an opposed activation 

 gradient. 



It is a question of some interest whether exogastrulation in veg,, with- 

 out or with micromeres, results merely from the large proportional size 

 of entoderm or from some other condition. From Horstadius' data it does 

 not appear that the micromeres play any considerable part in determining 

 exogastrulation. In this respect Figure 147 appears somewhat misleading, 

 but it does indicate a greater degree of exogastrulation with larger ento- 

 derm and less ectoderm with increasing number of micromeres. Even if 

 the micromeres increase concentration of vegetal substance, it is not evi- 

 dent how such increase determines failure of entoderm to invaginate. 

 Since veg^, with or without micromeres, is retarded in development and 

 about half the number isolated die early, inhibition, rather than the micro- 

 meres, is probably the factor determining exogastrulation; but whether 

 early isolation, reconstitution of ectoderm, or some other factor is the 

 inhibitor remains uncertain. If these isolates are inhibited, exogastrula- 

 tion is probably determined in the same way as with chemical agents. 



After implantation of four micromeres between an^ and an., of whole 

 embryos gastrulation and entoderm formation from presumptive ectoderm 

 occur at the region of implantation, as well as basally ; and plutei with two 

 guts and usually with extra skeletal elements result. Results are essen- 

 tially similar after implantation of four micromeres between an, and veg,, 

 except that the induced entodermal invagination is nearer the original 

 entodermal pole. Implantation of four micromeres in the apical pole of 

 whole embryos may induce a second small entodermal invagination in 

 that region, persisting in the apical region of the pluteus (Fig. 148, A-C), 

 and in some cases extra skeletal elements develop. Implantation of four 

 micromeres into the apical pole of apical half-blastulae results in a wide 

 range of forms. In some individuals development of the apical region is 

 more or less inhibited, but the original polarity persists; in others the 

 implanted micromeres may give rise to extra skeleton in the apical region ; 

 in still others a small apical invagination is induced, and another occurs 

 in the basal region of the apical half, that is, from presumptive ectoderm ; 

 and finally, in some there is a small apical invagination with complete 



