EMBRYONIC RECONSTITUTIONS 509 



alteration appears essentially similar to the activation of levels of the hy- 

 droid stem or the planarian or annelid body after isolation to a condition 

 making possible hydranth or head development. 



On the other hand, removal of the higher levels of the primary gradient 

 may result in earlier development and greater extent of the secondary 

 gradient. The forms with too large gut and excess skeleton and the 

 exogastrulae probably represent cases in which the secondary gradient 

 has "gained the upper hand" to greater or less extent and partly inhibited 

 or obliterated the primary gradient. This series of forms is interesting in 

 comparison with the series of inhibited head forms in planarian reconsti- 

 tution. There the inhibition of head development results from the activa- 

 tion following section at the posterior end of the piece. In the basal sea- 

 urchin half it apparently results from activation and development of a 

 secondary gradient in the basal region, but the effects on the primary 

 gradient show interesting similarities in the two cases. 



Reconstitution of isolated vegi and veg2 (see Figs. 145, 147) has also been 

 followed by Horstadius (1935). The veg^ ring represents the most basal 

 third of presumptive ectoderm and may develop as an entirely ectodermal 

 blastula but less extremely apical than the apical half; or it may reconsti- 

 tute primary mesenchyme and entoderm, gastrulate, and sometimes ap- 

 proach normal pluteus form, but development is slow. Horstadius ac- 

 counts for the differences in development by the assumption of variation 

 of the third cleavage plane from equatorial to subequatorial, so that 

 vegi is more basal in some individuals than in others. There may be such 

 variation, but the assumption is necessary only for the hypothesis of 

 overlapping substance gradients. In terms of primary and secondary met- 

 abolic gradients the range of variation in form is accounted for by quan- 

 titative differences in metabolism and in degree of dominance and of basal 

 activation, either characteristic of different eggs or resulting from differ- 

 ences in experimental procedure and environmental conditions. 



Isolated veg2 rings, though entirely entodermal in normal development, 

 reconstitute ectoderm but do not develop apical tuft or stomodeum which 

 are characteristic of the more apical ectodermal levels. Apparently the 

 activation in the primary gradient following isolation is less than in veg^, 

 again a parallel to hydroid and planarian reconstitution. Here, again, the 

 vegetal gradient does not "gain the upper hand," though it might be ex- 

 pected to do so, according to Horstadius' interpretation of various other 

 cases. However, these veg2 rings develop relatively large entoderm and 

 some become exogastrulae; but skeletal development is slight, probably 



