426 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



In relation with the cytochemical resuhs mentioned p. 360, the action of com- 

 pounds bearing SH- or SS-groups is especially interesting. Brachet (in Rapkine 

 and Brachet, 1951) insisted on the efficiency of reductors such as cysteine and 

 thiomalic acids for the neuralization of ventral explants. He consistently an- 

 swered the criticism expressed by Holtfreter (1945) and maintained that the 

 results were not caused by cytolysis. The opposite action of such oxidants as 

 alloxan was also emphasized, against Holtfreter. These experimental results are 

 in agreement with the observation data mentioned p. 360 and favor the idea that 

 macromolecules bearing SH groups are especially important in the process of 

 induction. This hypothesis is supported, although indirectly, by Lallier's resvilts 

 (1954) who stained the blastopore lip with dyes thought to combine with ribose- 

 nucleic acid (acridine, orange, toluidine blue, acriflavine), and nevertheless 

 obtained positive results. 



It would be tempting to introduce here the one known case of induction in 

 vitro by a definite organic substance. It could be permissible to present under this 

 aspect the discovery of Wilde (1955a, b) who demonstrated certain neural crest 

 characters in the ventral epiblast of advanced gastrulae, or of neurulae, by 

 addition of phenylalanine to the medium. Remarkably enough, the chordo- 

 mesoblast of the archenteron roof could be shown to act as the normal source 

 of this amino-acid (Wilde, 1956). However, if the neural crest primordium is 

 indeed a result of primary induction, the author does not explicitly refer to this 

 process, but more to the later appearance of the various neural crest derivatives in 

 correlation with the presence of endoblast or true mesoblast. This differentiation 

 falls more into the category of secondary inductions and will be considered in this 

 connection. Moreover, the experiment consisting of submitting ectoblast of the 

 early gastrula to the same conditions mentioned here does not seem to have been 

 performed. 



Thanks to the physical and chemical means just discussed, some more information 

 has been revealed concerning the process of primary induction. The young 

 (indifferent) ectoblast can be influenced in two opposite ways. It can be caused 

 either to perform notogenesis, or it can be caused to build up, often in the absence 

 of any mesenchyme or mesoblast, a thick-layered brain piece, eventually acquiring 

 the characters of an acrencephalon. 



The first transformation can be obtained only by a moderate centrifugation 

 of the fully segmented egg. The second may be attained by various means: 

 cultivation in a medium of high pH and/or low in Ca^^; short contact with the 

 anterior part of the notochord; or more prolonged contact with marginal material 

 or archenteron roof parts previously submitted to heating or to alcohol. 



It is important to decide which of these results expresses the tendencies nearest to 

 the normal ectoblast. In my opinion, it is the shift to notogenesis, for this tendency 

 is evidently present in the marginal zone, and this territory is in obvious continuity 

 with the ectoblast. The delimitation has only a "presumptive" value and can, 

 by micro-surgery, be shifted up or down. Also, centrifugation, at the speed which 

 has been used, is not traumatic, but only concentrates components of equal or 

 subequal density, a condition which probably allows them to exert, as Pasted s 



