Teleosts and Birds 



305 



a ventral half-blastula deeply stained with 

 neutral red, the resulting embryo arises 

 from the ventral pole, i.e., in reversed orien- 

 tation. However, if a stained ventral trans- 

 plant replaces the host ventral half, an 

 unstained axis arises from the dorsal pole in 

 the original direction. Thus staining seems 



dulus, according to Oppenheimer, no grafts 

 of lateral or ventral germ ring in gastrula 

 stages have been found to form axial tissues 

 unless implanted directly into the embryonic 

 shield, where inducing power is already 

 known to exist. Thus in both forms, com- 

 petence to form nervous system exists all 



Fig. 115. Comparison of distribution of inducing potency (vertical lines) and neural competence (hori- 

 zontal lines) in Salmo, Fundulus and the bird, during gastrulation. Approximately comparable stages are 

 shown for each form: blastula, early gastrula, late gastrula. In Salmo (Luther, '35, '36a,b, '37) all sectors of 

 the germ ring can form axial structures in grafts, at least part-way through gastrulation: this is interpreted as 

 showing even distribution of iaducing potency in early stages, normally expressed only at the dorsal pole. 

 This potency is gradually lost in ventral and lateral sectors as gastrulation proceeds. In Fundulus, no data 

 for the blastula are available. No data show inducing power outside the embryonic shield in gastrulation 

 stages (Oppenheimer, '34, '35, '36a, '38). In both forms, the capacity to respond to induction is apparently 

 not restricted to any locality during gastrulation. 



In the bird, all sectors of the blastoderm at the blastula stage (Lutz, '48) can form axes; hence both 

 potencies must be evenly distributed. Data from the chick (Waddington, '32, etc.; Woodside, '37) show that 

 the primitive streak or its precursors can induce axes throughout gastrulation, and that all ectoderm (includ- 

 ing extraembryonic) can give the neural response until fairly late in gastrulation, when this response 

 becomes gradually restricted to the prospective medullary region as suggested by the bottom figure of the 



only to reverse polarity and convey domi- 

 nance in an undisturbed ventral half-blas- 

 tula. Devillers emphasizes the relation be- 

 tween periblast and blastoderm and tends 

 to look on the former as a repository, or 

 chief effector, of the hypothetical "organiz- 

 ing substances" postulated by Tvmg (see 

 p. 300). 



As gastrulation begins, the latent capacity 

 for "self-organization" drops sharply in ven- 

 tral sectors, in the trout, and disappears 

 entirely before mid-gastrulation. In Fun- 



around the blastopore, as well as centrally, 

 during the blastula and at least in the early 

 gastrula stages; while inducing power, overt 

 or latent, becomes gradually limited to the 

 dorsal lip in Salmo; in Fundulus its exist- 

 ence has apparently never been demonstrated 

 outside the embryonic shield sector. 



Luther observes that the loss of potency 

 to differentiate axial structures from ventral 

 grafts can by no means be explained by 

 migration of a localized cellular material 

 from the ventral lip region. Convergence 



