468 PATTERNS AND PROBLEMS OF DEVELOPMENT 



planted in different orientation in relation to the host axis.'^ With vitally 

 stained inductor implants, oriented transverse to the host axis, Lehmann 

 observed that invagination began in the direction of the implant axis but 

 changed its direction toward the anterior end of the host. But when the 

 implant axis was directly opposed to the host axis, direction of invagina- 

 tion was highly variable, and the axis of the secondarily induced embryo 

 might be more or less opposed to that of the host embryo. In such cases 

 the secondary neural plate was broad and often did not close normally, 

 and the death rate was high. Apparently there is in the host a directive 

 action of some sort, influencing the direction of invagination and migra- 

 tion of the implanted inductor; and when orientation of implant and in- 

 duced axis are opposed to the host axis, form and development of neural 

 plate suggests, as Spemann has noted (1936, p. 97), that it is opposed by 

 some factor in the host. In these cases two opposed polarities are involved, 

 one established, the other introduced by experiment. Since the neural 

 plate in normal animals shows a general anteroposterior gradient in early 

 stages, it is probably that the inductor implanted with axis opposed to the 

 host axis induces in some cases a new gradient opposed to that of the host, 

 but this does not attain full development. These results are apparently 

 very much like the relation of the original gradient to the new gradient re- 

 sulting from section at the proximal ends of stem pieces of Tuhularia and 

 Corymorpha; the proximal hydranth gradient is shorter than the distal, 

 the scale of organization of the proximal hydranth is smaller than that of 

 the distal, and its development is slower. Ganglionic planarian grafts in 

 posterior regions often show similar differences in induced reorganization 

 posteriorly and anteriorly. The alterations in direction of invagination of 

 the amphibian inductor implant by the host are also closely paralleled in 

 ganglionic planarian grafts ; the polarity of the graft may be altered to con- 

 form with that of the host, particularly with implantation in the head re- 

 gion, but at other body-levels the polarity of the implant usually persists 

 (p. 382). Frequency of induction by Corymorpha implants depends on 

 level of origin of implant, level of implantation, and dominance of the host 

 hydranth (p. 378). At present interpretation in terms of gradients in- 

 volving dynamic factors serves as well for these relations in amphibia as in 

 hydroids and planarians. 



HOMEOGENETIC INDUCTION 



Induction of a neural plate by chorda-mesoderm is heterogenetic in 

 character, that is, something different from the inductor results from the 



'' Geinitz, 1925c; Spemann, 1931; Lehmann, 1932. 



