EMBRYONIC RECONSTITUTIONS 511 



ditions — perhaps contact of the basal region with bottom of the container, 

 resulting in a decrease in the oxygen supply — a small entodermal region 

 may be determined. Primary and secondary gradients are more nearly 

 balanced in isolated vegi than in isolated whole ectoderm; but in terms 

 of primary and secondary activity gradients the balance results from the 

 changes following isolation rather than from pre-existing concentration 

 gradients. It was noted earlier that Horstadius' interpretations often seem 

 to be in terms of metabolism, rather than of concentrations, and to imply 

 something very similar to dominance. 



Apical halves with the ring veg2 added gastrulate before forming mesen- 

 chyme but develop into plutei with relatively small entoderm, although 

 only two-thirds of the presumptive ectoderm and the whole presumptive 

 entoderm are present. Horstadius accounts for the small size of entoderm 

 by the fact that the presumptive entoderm also forms mesenchyme. Mes- 

 enchyme formation, however, occurs late, "at the end of invagination," 

 when the size of the entoderm must be more or less determined; and it 

 seems improbable that the small number of cells forming mesenchyme 

 would decrease entodermal size very greatly. It is suggested as another 

 interpretation that in this case vego, being brought into direct relation with 

 more apical levels of the primary gradient, is partly ectodermized. 



Whole ectoderms (anj, a«2, vcgi) with four micromeres added basally 

 differ markedly in development from whole ectoderms alone. They give 

 rise to plutei, some scarcely distinguishable from controls, but most with 

 entoderm somewhat too small. In this combination presumptive ento- 

 derm is absent, but entoderm of normal or almost normal proportions is 

 reconstituted. Apparently the implanted micromeres bring about this en- 

 todermization of presumptive ectoderm, as they do when implanted else- 

 where in ectodermal regions. Horstadius holds that they increase the veg- 

 etal gradient, presumably by producing vegetal substance; von Ubisch, 

 that they attract vegetal substance. There is also the possibility that, 

 whether they are or are not specifically different from other cells in early 

 stages, they initiate basal activation and the secondary gradient, which 

 is apparently a factor in gastrulation and differentiation of entoderm. 

 They may even be more effective in this way in consequence of isolation 

 from their original relations to other cells. 



Apical halves with vcg, and micromeres added include all the presump- 

 tive entoderm but only two-thirds of the presumptive ectoderm. Thev 

 give rise to plutei with somewhat too large entoderms and thus differ 

 from apical halves plus veg2, in which the entoderm is small. Here, again, 



