PRIMARY INDUCTION 



EXPERIMENTAL DATA 



393 



diencephalon, paraphysis; {4) telencephalon, i olfactory placode, paraphysis, ganglion; 

 (5) telencephalon, complex nasal placode, paraphysis, ganglion. Among the 13 Axolotl 

 embryos, 4 should be discarded here, for one strip carried some "mesoderm cells", and 

 the two others produced tail-like formation, indicating that the strip was cut too low and 

 included presumptive tail material. Among the 10 others, we read: (/) atypical neural 

 mass; {2) the same; (j) prosencephalon, i nasal placode, a rudimentary retina; {4) a 

 telencephalon, nasal placodes, paraphysis; (5) the same; {6) diencephalon, i eye, i lens; 

 (7) badly-infected neural mass; {8) tel-and diencephalon, i nasal placode, 2 paraphyses, 

 eye, lens, epiphysis; [g) prosencephalon, i nasal placode, i atypical eye; {10) prosence- 

 phalon, 2 nasal placodes, 2 eyes. Altogether, this makes out of 19 valid cases, 13 with 

 ascertained manifestations of prosencephalon. If the untabulated experiments are taken 

 into consideration, it appears that an appreciable number of these early transplants have 

 not shown induction in the folds. Of course, the main interest lies in the positive cases, 

 but we must insist here on a consideration of the negative ones to avoid the impression, 

 probably false, that the "activation" is quite a sudden process. The above enumeration 

 follows Giladi's order, in which the strips she presumed the more caudal are placed last. 

 It can be seen, especially for the axolotl, that these inductions are the best ones. This 

 could mean that they had time enough to get the best from the inductory flow. On the 

 other hand, if the assumed location is exact, the contact with the migrating archenteric 

 roof, and especially with its cephalic part, could only be transitory. This is indeed a 

 remarkable fact, and worth looking into the question of how such a process begins. "In 

 the early gastrulae (p. 187) the implanted folds remain unchanged for 2 or 3 days, after 

 which period the basis of the fold began to thicken" (Fig. 61, b). Such an appendix was 

 very fragile, and often remained unchanged for 1-2 days, then progressively disintegrated 

 from top to base. However, in 2 cases which were fixed on the third day after an early 

 operation, at the moment disintegration began, an irregular mass of neural tissue, with 



Fig. 61. Two examples of a ventral fold created in the most reactive species, the axolotl. 

 (a, b) First case, (c-e) second case, (a) Shortly after the operation; (b) two days later, with 

 the characteristic swelling at the base of the fold ; (c) the young gastrula before the operation; 

 (d) the embryo three days after the operation, still showing a dorsal defect; the tip of the 

 thickened fold begins to desintegrate; (e) section through the base of the fold fixed at this 

 stage; the inner cells have a distinct neural appearance, even forming a mass with two 

 cavities, suggesting a brain (Eyal-Giladi, 1954). 



Literature p. 483 



