Ill 



SECONDARY AND MINOR INDUCTIONS 



455 



I H N 



O-FLUOROPHENYL ALANINE 



M-FLUOROPHENYL ALANINE 

 H H 



P-FLUOROPHENYL ALANINE 



1 ADDITION TO PHENYL GROUP 



2 REPLACEMENT OF PHENYL 

 GROUP 



3 REPLACEMENT OF a AMINO 

 GROUP BY HYDROXYL 



4 NORMAL METABOLITES 



PHENYL ALANINE 



A<^i 



V 



H H 

 I I 

 C— C-COOH 



I I 



V\ 



H H 

 B-3 THIANAPHTHYL ALANINE 



3-AMINO TYROSINE 



H H 

 I I 

 -C— C— COOH 



I I 

 H N 



B-2 THIENYL ALANINE 



H H 

 I I 

 •C -C-COOH 



H N 

 X3/ H^^ 



B-3 THIENYL ALANINE 



H H 

 I I 

 -C— C— COOH 



I I 

 H N 



/\ 



H H 



B-2 FURYL ALANINE 



PHENYL LACTIC ACID 



Fig. 93. Synoptic table of the amino-acid analogues utilized to modify the medium in 

 which the explants of neural crest have been reared. From Wilde, 1956b. 



differentiation. The replacement of the amino acid by its analogues (Fig. 93) was 

 tested. The recurrence of certain types of cells and the inhibitory exclusion of 

 others could be correlated with the presence of the phenyl ring and/or of the 

 amino group of the molecule. Within the framework of the anti-metabolite theory, 

 admitting that each analogue stops some stage of metabolism by competing with 

 a specific enzyme, it becomes plausible that the mild strtictural diflference intro- 

 duced "leads to immobilization of each enzyme which acts upon it." Accordingly, 

 melanogenesis and the pigment load in the cells depends on "the availabihty of 

 sufficient a-amino groups on molecules of the configuration of phenyl alanine or 

 tyrosine." Alternatively, "unchanged phenyl groups appear to be essential to 

 chemical processes of ecto-mesenchyme differentiation". 



(j) Ventral epiblast of postgastrular stages was cultivated {ibid.) in the basal 

 medium, supplemented by phenyl alanine (1.0-8.0 mmols/1). Under this influence, 

 the explant ceases to expand "as a broad sheet of mutually adherent cells". The 

 cells disperse in a typical ectomesenchyme fashion; after some i-io days spent as 

 amoeboid cells, they become spindle-shaped or stellate "with slender filopodia", 

 and melanogenesis occurs. Such transformation of the ventral epiblast, formerly 

 thought to be nearly determined, means that it possesses beforehand the enzymatic 



Literature p. 48'} 



