EMBRYONIC INDUCTORS AND ORGANIZERS 469 



induction. But induction of parts like the inductor — homeogenetic induc- 

 tion — also occurs. Implanted pieces of the dorsal inductor may dominate 

 adjoining host tissue, even of a different species, and induce mesoderm 

 formation from it, with resulting development of a complete chorda-meso- 

 derm; in heteroplastic transplants this consists in part of one species- 

 tissue, in part of another.'^ As noted above, presumptive ectoderm, 

 whether neural plate or epidermis, transplanted to the presumptive meso- 

 dermal region, invaginates with it and becomes mesoderm. Here, again, 

 the similarity to dominance in invertebrate reconstitution appears. A 

 piece of inductor dominates adjoining regions of the host, and they become 

 parts of the system concerned. These cases have been regarded as homeo- 

 genetic induction, but it may be questioned whether they are really ho- 

 meogenetic except in the general sense that mesoderm induces mesoderm. 

 The regions induced become parts of the chorda-mesoderm system; but 

 they supplement, rather than duplicate, the parts already present, so that 

 a harmonious whole results; consequently, they must become different in 

 some way, either in gradient-level or specific constitution, from the parts 

 of the system present in the implant. Mesoderm induced by mesoderm 

 from presumptive ectoderm is also capable of inducing neural plate and is 

 not species-specific in action.'*^ Presumptive neural plate or epidermis, 

 transplanted to entodermal regions, becomes entoderm. In some of these 

 cases it is perhaps not entirely certain whether or to what extent induction 

 by the region receiving the implant or the general physiological environ- 

 ment is concerned in determining the result. 



A case of homeogenetic induction concerning which there is no doubt is 

 the induction of neural plate by neural plate after it has itself been induced 

 by chorda-mesoderm. Not only neural-plate naterial of the normal em- 

 bryo but that of a supernumerary neural plate induced by implanted in- 

 ductor can induce neural plate when implanted in the blastocoel, and its 

 action is not species-specific."' This result, although unexpected by the 

 investigators (see Spemann, 1936, p. 137), and justifiably so if the inductor 

 is primarily specific in its action, is exactly what might be expected if the 

 inductor is primarily an activator, whatever the manner in which activa- 

 tion is brought about. Neural-plate material has supposedly become dif- 

 ferent in some way from chorda-mesoderm ; but if induction is by means of 

 a specific substance, both must possess or produce it, and neural-plate ma- 



'8 Spemann und H. Mangold, 1Q24; O. Mangold und Seidel, 1927; Lehmann, 1932; B. 

 Mayer, 1935. 



''O. Mangold, 1923; Spemann und Geinitz, 1927; Holtfreter, 19336; Raven, 19356. 

 2" O. Mangold und Spemann, 1927; O. Mangold, 1929c. 



