400 



GERMINAL ORGANIZATION 



INDUCTION PHENOMENA 



During the next year, Deuchar (1953) needed controls for her dissociation ex- 

 periments (p. 422), so she wrapped in young ectoblast the first, second, and last 

 third, respectively, of the archenteron roof of the late gastrula, the neurectoblast 

 having been peeled off. A valuable innovation in her experiment was that she 

 looked for a comparison between the inducing effect of i explant and 3 others of 

 the same origin. The main results are summarized in Table 2. 



TABLE 2 



INDUCING EFFECT OF THE ARCHENTERON ROOF 



Each piece simple or triplicated 



Statistically, significant differences appear in the number of inductions of each type 

 produced by the successive thirds, especially when 3 pieces were used together. The 

 general inducing activity is the highest for posterior parts of the roof, but the acrencephaHc 

 character, revealed by the eye frequency, is more often manifested by the middle parts, 

 in agreement with Mangold's and Ter Horst's results. Trunk and tail characteristics are 

 reinforced with posterior parts. However, no group specifically induces the acrencephalon 

 or the suprachordal cord. This absence of definite regionality inust be applied with 

 caution to the normal roof of the archenteron. First, conditions in explants are, as already 

 noted (p. 399) somewhat different from blastocoele insertions. Second, the division of the 

 inducing system may be important in itself. The author expresses the interesting suggestion 

 that "the anterior third of the organizer normally depends upon the two other thirds for 

 supplies of essential substrates". Whe shall come back to this idea (p. 473). 



In 1953, Takaya re-examined the modification of potency correlated with 

 invagination. His new experiment differed from the one of Okada and Hama 

 only in the fact that he did not use the blastoporal lip itself but two neighboring 

 areas, one uninvaginated, the other already incorporated in the roof of the 

 archenteron. They were wrapped in competent ectoblast and reared for 3 more 

 weeks. The interest lies in the early stages of gastrulation, previous to the large 

 yolk plug stage. 



The results were very clear cut. All surface fragments, even if they included the pre- 

 sumptive prechordal plate, yielded notochord, which is not astonishing, but remarkably 

 they induced chordal parts of the neural axis, except in i case. The same material, when 

 recently invaginated did not differentiate into notochord (only in i case out of 26) and 

 induced a forebrain in 9 of the 26 cases, twice including eye; most other cases formed 

 a neural mass or a spinal cord. This result is in good agreement with the insertions in 

 the blastocoele (p. 375). As in the latter, the influence of age was examined. Explants 



