EMBRYONIC INDUCTORS AND ORGANIZERS 467 



of fusion. Fusion of the anterior half of such an explant with the anterior 

 end of a whole explant gives similar results. Lateral fusion with antero- 

 posterior axes in opposite directions does not give neural tubes. More- 

 over, a single explant can develop neural tube if lateral edges are brought 

 together to form a tube having an anteroposterior axis. If anterior and 

 posterior margins are united, a neural tube rarely develops. Barth holds 

 that neural-tube development in these explants results from maintenance 

 of the gradient pattern by proper fusion and healing. If the gradient is 

 obHterated or weakened by the fusions, epidermis, instead of neural tube, 

 results. In view of all the evidence it appears probable that any determi- 

 nation or Bahmmg toward determination in the presumptive neural plate 

 independently of the inductor is predominantly quantitative and highly 

 susceptible to change in environment. 



Presumptive chorda-mesoderm is apparently somewhat more stably 

 determined. Pieces transplanted to other regions of embryos of the same 

 stage as the donor usually invaginate, form chorda-mesoderm, and induce; 

 but under certain conditions this region may take part in formation of 

 ectoderm.'''' The apparently more advanced determination of this tissue is 

 similar to the relatively high stability of high gradient-levels of hydroids, 

 planarians, etc., when transplanted to other regions. They are more able 

 to persist and induce than material from other gradient-levels (chap. xi). 

 The presumptive inductor region behaves in transplants exactly as would 

 be expected if it represents a sufficiently high gradient-level to render it 

 more or less independent of other parts. The question of its metabolism 

 has already been discussed (pp. 153-58). 



ORIENTATION OF THE IITOUCED EMBRYO IN RELATION TO THE HOST 



The longitudinal axis of the neural plate induced from presumptive 

 epidermis by implanted inductor tissue coincides with that of the implant, 

 and this, in turn, is definitely related to the direction of invagination and 

 migration of the chorda-mesoderm material. When invagination in the 

 normal direction in the intact embryo is prevented by injection of gelatin 

 into the blastocoel, it may occur in the opposite direction across the basal 

 region, and the neural plate develops there. Under these conditions part 

 of the inductor region may fail to invaginate, and neural plate may be in- 

 duced in it; also, part of the presumptive neural plate may become epi- 

 dermis (Eakin, 1933, 1939a). 



The secondary embryonic axis induced by inductor implants is very 

 commonly parallel to the host axis, even when the inductor tissue is im- 



'<> Vogt, i922(z; Mangold, 1923; Mangold und Seidel, 1927; Bruns, 1931; Lopaschov, 1935. 



