Ill 



PRIMARY INDUCTION 

 . otyp. ect 



EXPERIMENTAL DATA 



389 



nr p 



nr f 



nr tis. 



Fig. 57. Fate of an ectoblast fold implanted as in Fig. 54. (A) an implant probably grafted 

 outside the neural area, or at the limit of this area; other cases yield atypical epiblast. (B) 

 an attachment to the prechordal region of an axolotl neurula, with the differentiation in 

 three zones, neural, mesectoblastic, ectoblastic. (C) hypothetical spreading of the activating 

 principle through the prospective neural and mesectoblastic areas of host and implanted 

 fold. atyp. ect., atypical ectoblast; bal., balancer; di-enc, diencephalon; d.m.L, dorsal 

 middle line; ect., ectoblast; ent., endoblast; f.g., foregut; impl., implant; lent., lentoid ; 

 mesect., mesectoblast; mes. enc, mesencephalon; n., neural groove; n.r.f, neural fold; 

 n.r.p., neural plate; nr. tis., neural tissue; p.p., prechordal plate; spr. act. pr., spreading 

 activity of the prechordal material ; vert, impl., vertical implant. From Nieuwkoop et al., 1 952. 



When the implantation happens to be located in the forebrain area, the base 

 soon takes on the same appearance as the neural plate, while the distal end may 

 remain atypical ectoblast (Fig. 57b). Sometimes the two zones are separated by 

 some ectomesoblast. The interpretation of these early aspects is that some 

 "activating" principle spreads "through the ectodermal layer, mainly emanating 

 from the dorso-median zone of the ectoderm where this ectoderm is in close 

 contact with the median portion of the underlying archenteron" (their p. 25). This 

 assumption (Fig. 57c) would explain why lateral implants are less modified than 

 median ones. The territory which is normally exposed to the activating influence, 

 and in which there is a maximum zone with a decrement in all directions is 

 called the neural field (Nieuwkoop et al., 1952). 



In 1954, the same author gave complementary information on the early behavior of the 

 folded transplants (Nieuwkoop and Nigtevecht, 1954, their p. 178). Using axolotl gastrulae 

 as donors, he implanted the graft with a sagittal orientation on the presumptive rhomben- 

 cephalon. Nieuwkoop insists that modifications of the external aspects take place and adds 



Literature p. 483 



