Ill EXPERIMENTAL MODIFICATIONS 423 



The general effect' of LiCl on morphochoresis has been studied by Lehmann 

 (1937) and especially by Pasteels (1945). The depressive result progressively 

 affects the cephalo-caudal and dorso-ventral gradients, without exhibiting any 

 difference of susceptibility between the acrencephalic part and the rest of the 

 embryo; on the contrary, some of the anomalies obtained override the limit of 

 the two regions. 



A further experimental analysis has been done, mostly in Urodeles, by three authors. 

 Hall (1942) cultivated in association parts of gastrulae obtained from normal and Li- 

 treated eggs. He concluded that the chordo-mesoblast has a higher susceptibility than the 

 reacting ectoblast. Gallera (1949) repeated the experiment described on p. 3-1 using 

 Li-treated donors. In the 8 cases where the action of both grafts (Fig. 42, p. 372) could 

 be compared, a general depression of induced structures was apparent. Especially grafts B, 

 containing the prechordal area, induced a rudimentary fore-brain only once, with a slight 

 optic expansion. The other cases produced only trunkal structures of a rather low level, 

 myotomes, pronephros, and limb buds. This result is in close agreement with the effects 

 obtained on whole embryos (supra) and means a real conversion of the inducing system. 

 Grafts containing chordo-somitic material at the exclusion of the prechordal territory 

 formed only a notochord of reduced size and a slender spinal cord. No explant disclosed 

 any acrogenetic tendency, a noticeable difference from similar, non-treated explants (p. 374) . 

 Lombard (1952) re-studied the Li-effects on Xenopus and several Urodeles. The novelty 

 in his work was that in Ambystoma and Pleurodeles he compared the effect of exchanging 

 the neurectoblastic areas of normal and Li-treated young gastrula. Both combinations, 

 either a normal gastrula provided with treated neurectoblast, or vice versa, behave re- 

 markably better than whole embryos. While these were effected with cyclopy, the operated 

 animals were only slightly micrencephal. Moreover, the degree of micrencephaly was 

 slightly more severe in the first combination, and the author asserts that this difference is 

 statistically significant. It would, however, be difficult to accept his conclusions that "the 

 lithium effect is to a greater extent caused by the action of the lithium ion on the ectoderm, 

 viz. the presumptive neural plate, than by its action on the archenteron roof" (Lombard, 

 1952, p. 51). The main disorders have indeed been observed in the chordomesoblast; 

 these phenomena of recovery, although interesting, cannot prevail against this fact^. 



The morphogenetic effect of urea must be considered, although it does not 

 directly concern primary induction. The addition of this chemical to the medium 

 causes essentially a chordalization of neighboring organs, e.g., the neural axis, 

 sometimes the somites, and more frequently the endoblast. A thorough analysis 

 by Fautrez (1951) concludes that a process takes place later than primary 

 induction, and probably consists in a diffusion of organ-specific substance(s) 

 elaborated by the differentiating notochord. This effect could be due to the 

 rather ordinary permeabilizing effect of urea. This idea would be consistent with 

 the prolongation of the period of competence which Gallera (1953) has provoked 

 in the aging ectoblast by this means. 



1 By using labelled Li*, it has been shown that the cation accumulates especially in the 

 dorsal part of the egg, and concentrates into the pigment granules (Ficq, 1954a). 



2 For LiCl, see also Kawakami (1953, 1955) and Ohtsuka (1956). This author has shown 

 that the hypomorphic effect of Li* is lessened by a subsequent treatment with 2, 4-dinitro- 

 phenol. This can however not be explained by an inhibition of glycolysis, for the concen- 

 tration of the inhibitor is too low for that. Its favorable action seems due to the arrest 

 of development, during which the absorbed Li* has time to abandon the egg by diffusion. 



Literature p. 483 



