482 PATTERNS AND PROBLEMS OF DEVELOPMENT 



or less differentiation without gastrulation (Oppenheimer, 19366). More- 

 over, up to the end of the blastula stage a piece from any region of the 

 blastoderm, including part of the margin, may, when isolated in the yolk 

 sac of an older individual, give rise to a more or less complete embryonic 

 primordium. Neural tissue develops in most cases, notochord almost as 

 frequently, even from extraembryonic regions containing none of the pre- 

 sumptive chorda-mesoderm; and very frequently gut and kidney also de- 

 velop. This capacity of parts of the blastoderm for independent reconsti- 

 tution decreases rapidly in extraembryonic regions with gastrulation, and 

 inductor action becomes necessary for embryo formation (Luther, 1936a). 

 In its high capacity for reconstitution in earher stages the teleost blasto- 

 derm differs from the amphibian blastula, although under certain condi- 

 tions neural, or even mesodermal, tissue may develop from presumptive 

 amphibian epidermis or neural plate, as noted above. Luther regards the 

 inductor as an activator, differing quantitatively, not qualitatively, from 

 other parts, and holds that the capacity of isolated pieces of the blastula 

 blastoderm to reconstitute embryonic primordia results from levels of 

 activity high enough in all regions at this stage to determine embryo for- 

 mation without aid of an inductor. As gastrulation begins, this activity 

 decreases in extraembryonic regions, and embryo formation occurs there 

 only by induction. This view agrees closely with that suggested above for 

 the amphibia, and the data on differential susceptibihty in teleosts also 

 show a high susceptibility in the inductor region as gastrulation ap- 

 proaches (p. 150). Perhaps the point of greatest interest in these experi- 

 ments on teleosts, as compared with those on amphibians, is the recon- 

 stitution of new inductor regions in isolated pieces, even from extraem- 

 bryonic regions without any of the original inductor. This does not sug- 

 gest any considerable specificity in the inductor. The fact that the extra- 

 embryonic regions do not develop inductors and embryos in normal de- 

 velopment suggests that the dominance of some other region, presumably 

 the more active embryonic region, or later the inductor region itself, pre- 

 vents such development, and that an activation resulting from isolation is 

 sufficient to bring about development of neural tissue or even of inductor 

 region. In short, the data give further support to the conception of neural 

 induction as primarily an activation and appear difficult to interpret in 

 other terms. 



Following division of the archenteric roof of the trout embryo into 

 anterior, middle, and posterior pieces, inclosure of each in indifferent ecto- 

 derm from the extraembryonic region, and implantation in the yolk sac, 



