470 PATTERNS AND PROBLEMS OF DEVELOPMENT 



terial of one species must provide the substance necessaty to induce neural 

 plate in another. But, if neural-plate induction is primarily activation of 

 ectoderm, plate material, having itself been activated, should be capable 

 of inducing neural plate within a certain range of its developmental stages. 

 Actually the capacity is present not only in the open neural plate but per- 

 sists after closure of the folds and probably up to hatching. There is ap- 

 parently no regional correspondence transversely between inducing parts 

 of the plate and the induced plate. The right half of the anterior third of 

 the brain region from an axolotl neurula with closed neural folds, implant- 

 ed in the blastocoel of a Triton gastrula, develops, according to its origin, 

 as a half-brain region but induces a bilaterally symmetrical, very complete 

 head (O. Mangold, 1932a). There is, however, a longitudinal differential 

 in inductions by neural plate. Brain and head structures are induced most 

 frequently by anterior parts of the plate, and, the more posterior the part 

 of the plate used as inductor, the more posterior is the character of the in- 

 duced structure (O. Mangold, 1933). Different levels of the plate, like 

 different levels of chorda-mesoderm, are apparently ''head-inductors" and 

 "trunk-inductors." Also, implants of caudal portions of the plate may 

 bring about development of a supernumerary tail with neural tube and 

 mesodermal somites, induced in host tissue (Bytinski-Salz, 1931; O. Man- 

 gold, 1932a). It appears, however, that the extreme posterior part of the 

 plate normally gives rise in part to mesoderm of the tail (Bijtel und Woer- 

 deman, 1928; Bijtel, 1931); consequently, its inducing capacity should be 

 similar to that of caudal mesoderm. 



THE PROBLEM OF THE NATURE OF THE INDUCING FACTOR 



Early in the course of investigation of amphibian induction by chorda- 

 mesoderm material the same question arose that had arisen earlier with 

 respect to other cases of dominance and induction in development, that is, 

 the question of the manner in which induction is brought about. One hne 

 of experiment concerned with this question was the attempt to determine 

 whether other tissues, not only of amphibians but of other animals, would 

 induce. Apparently first was the discovery that the developing amphibian 

 limb bud had some inducing power (O. Mangold, 1928); the regenerating 

 tissue of the urodele leg and tail gave clear-cut induction (Umanski, 1932, 

 1933). These cases suggest a relation between induction and tissue activ- 

 ity. On the other hand, narcotized dorsal-lip tissue was found to be effec- 

 tive (Marx, 1930). However, the whole problem of induction began to ap- 

 pear in a new light when it was discovered that the urodele dorsal lip and 



