A series of explantation and implantation experiments has been carried out in order to discover if the amphibian organizer is able to induce normally differentiated nervous tissue after disaggregation and reaggregation of its cells.

Explants

1. In twenty-one control explants of ectoderm alone, no neural tissue was formed. This made it certain that the inductions in all the experiments were due entirely to the organizer tissue.

2. From a small number of explants of early gastrula dorsal lip into early gastrula ectoderm it was confirmed that, as Holtfreter has claimed (1944b), the regional inducing power of this tissue is in no way inferior after disaggregation of its cells. The numbers of brain, eye and neural tube inductions (Table 1) did not differ significantly as between experiments and controls, and these tissues were often asymmetrical in both series.

3. The archenteron roof from the late gastrula also showed no deterioration in regional inducing powers after disaggregation of its cells. When the percentages of brain, eye and neural tube inductions were compared as between experiments and controls, no significant difference was evident.

4. Combinations of three anterior, three middle or three posterior thirds of the archenteron roof in a control series gave different frequencies of brain, eye and neural tube inductions (Table 4). However, these regional differences were no longer apparent when the separate thirds had been disaggregated before explantation (Table 5). This was apparently due to a reduction of the number of brain inductions by the two anterior thirds.

5. Since no regional differences were detectable in control explants of single thirds of the archenteron roof (Table 3), there seems to be some size factor limiting the range of inductions in small explants. The influence of the quantity of inducing tissue is discussed in the light of Marx's (1925), Dalcq & Lallier's (1948) and Lopaschov's (1935) results.

6. Owing to the frequent asymmetry and the incomplete differentiation of the nervous tissue in explants, some implantation experiments were also carried out. Disaggregated dorsal lip tissue from an early gastrula was implanted into a ventral position in an early gastrula host. It appeared to induce just as successfully as the normal dorsal lip. The secondary embryos of both experiments and controls were usually incomplete. In the experiments they were more complete if they lay near the host's axis, and if the implanted tissue had invaginated for the maximum distance. The possible reasons for this relationship are discussed.

7. In the secondary embryos, the formation of symmetrical, paired somites was always associated with the presence of an archenteron and a notochord. Explants containing no endoderm never form an archenteron, and in the present experiments they also only rarely formed somites.

8. It is concluded that these results confirm Holtfreter's findings by showing that the complete organizer, even at the late gastrula stage, can differentiate normally itself, and can induce regionally differentiated nervous tissue, after its cell arrangement has been upset by disaggregation. But the results of series 4 show that specific regional properties are no longer demonstrable in separate thirds of the archenteron roof after this treatment.

9. Comparison of explants and implants leads to the conclusion that the completeness and symmetry of all inductions depends on the final layout of the organizer tissue when it begins to induce. The possible influence of the host endoderm on this layout is discussed.

10. It has been shown that all the quantitative data can be explained by postulating the existence of an anterior-posterior gradient in the concentration of a single evocator. Although results of other workers show that qualitatively different inducing substances must exist, there is no reason for rejecting the idea that regional differentiation depends partly on quantitative differences.

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