440 PATTERNS AND PROBLEMS OF DEVELOPMENT 



the long cilia disappear, and they become uniformly ciliated spherical 

 forms (Fig. 146, C). Some apical halves, however, show less extreme api- 

 cal partial development (Fig. 146, D), and some may finally develop 

 ciliated band and stomodeum (Fig. 146, E). A.pical halves differentially 

 inhibited by lithium salts develop mesenchyme and entoderm (von 

 Ubisch, 19256, 1929) and occasionally do so in supposedly normal environ- 

 ment (von Ubisch, 1936a), though these cases may perhaps represent slight 

 differential inhibition by some uncontrolled factor. Evidently their fail- 

 ure to form mesenchyme under the usual conditions is due not to lack 

 of potency but to a scale of polar organization longer than the piece. 

 When the scale is decreased by lithium, basal parts develop. The case is 

 completely parallel to reconstitution of short Tuhularia and Corymorpha 

 pieces in relation to scale of organization (pp. 345-49). Animal halves 

 with four micromeres implanted basally develop into practically normal 

 plutei (Fig. 146, F-H); that is, the vegetal gradient is increased by the 

 micromeres (Horstadius), or the primary gradient is partly obliterated 

 and reversed by the activation of the micromeres. 



Figure 147 indicates results of implanting one, two, or four micromeres 

 in the isolated blastomere rings. Afii requires four micromeres for normal 

 development; a«2 with one micromere approaches pluteus form, is normal 

 with two micromeres, and less completely developed apically with four. 

 Development of vegi is not complete in any case but becomes less com- 

 plete as more micromeres are implanted, and entoderm is relatively "too 

 large"; veg2, with one or more micromeres, develops exogastrulae with 

 large entoderm and small ectoderm, 



Horstadius admits that this schematic representation is not entirely 

 adequate, and von Ubisch (1936a) has called attention to the selection of 

 particular experimental results on which it is based. Assuming, however, 

 that it does show, in a general way, effects of difference in apicobasal level 

 of blastomere rings and of number of micromeres present, it indicates, as 

 regards an^, awa, and vegi, that the effect of implanted micromeres in- 

 creases with the number implanted and with increasingly basal level of 

 the blastomere ring. If the micromeres undergo activation sooner or later, 

 as they apparently do preceding normal gastrulation, they probably oblit- 

 erate the primary gradient and reverse gradient direction over a greater 

 or less distance from the region of implantation; and this secondary gra- 

 dient extends farther and is more effective at lower than at higher levels 

 of the primary gradient, that is, in the more basal blastomere rings. 

 Activation of micromeres increases the scale of basal or vegetal organiza- 



