454 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



Of the Other manifestations which appear in the brain, epiphysis and paraphysis 

 must be mentioned. Both are derivatives of the prosencephalon and are to be con- 

 sidered as delayed inductions. In Discoglossus at least, an isolated epiphysis may be 

 obtained. A detailed analysis is due to Van de Kamer (1949) by experiments on 

 the neural plate stage. This author considers the paraphysis as a "relatively lower 

 grade of segregation" compared to epiphysis. 



{b) Neural crest and derivatives 



The limited space unables us to discuss some important contributions as those 

 of Twitty (1944, 1945), of Twitty and Bodenstein (1944), of Rawles (1955), of 

 H. Lehmann (1952). Lehmann and Young (1952) and of Niu (1954). Let us 

 restrict ourselves to two aspects of direct theoretical interest for the understanding 

 of induction. The first concerns the biochemical factors permitting and even causing 

 the realization of the neural crest potencies as demonstrated by the previously 

 mentioned (p. 426) experiments of Wilde (1955a, b). The demonstration pro- 

 ceeded in three elaborate steps. 



(j) An appropriate nutrient "basal" medium, free of exogenous proteins, was 

 chosen, composed of half-strength Holtfreter's solution, glucose, sodium nucleinate, 

 agar and ultrafiltrated serum. The lateral rim of the cerebral plate of Ambystoma 

 mac, i.e., the material containing the neural crest cells, was excised and cultivated 

 in this medium. The type of cells which differentiated was recorded for these 

 tissues explants either alone or combined with the following tissues: true meso- 

 blast, stomodoeal epiblast and foregut endoblast taken together; foregut endoblast 

 alone or stomodoeal epiblast and true mesoblast together. These histiotypic 

 cultures established first that the explanted neuro-epithelium by itself produced 

 neuroblasts and pigmented ectomesenchyme. With the adjunction of foregut 

 endoblast, procartilage and cartilage proceeded from the same neuro-epithelium.^ 

 If stomodoeal epiblast was added to this, teeth were obtained. With these com- 

 binations, some striated muscle may also appear. True mesoblast "has no effect 

 on the appearance of differentiated head organs and tissues in vitro, beyond an 

 increase in the amount of striated muscle" (p. 588). In short, all the known de- 

 rivatives of the neuro-epithelium^ are obtained in this basal medium, which 

 means that its composition is completely reliable (Wilde, 1955a). 



(2) Starting from the suggestion of Willmer (1951) that neural crest cells may 

 have a characteristic metabolism concerned with tyrosine, and considering that 

 in vertebrates tyrosine is normally replaceable by phenyl-alanine, the influence of 

 analogues of the latter was studied (Wilde, 1956a, b). The addition of i.o mmol/1 of 

 phenyl-alanine (or tyrosine) to the basal medium did not alter the histiotypic 



^ This is in good agreement with other works bearing on the branchial region, for inst. 

 T. S. Okada (1956, 1957), Mangold (1957). Mangold has also found that endoblast is an 

 inductor of the branchial structures, and that the anterior part of the endoblast induces the 

 heart. 



^ Seno and Nieuwkoop (1958) have re-examined the autonomous and dependent differentia- 

 tion of neural crest in newt embryos. Corium is obtained if epiblast is available. No lep- 

 tomeninx nor ganglions differentiate if no central nervous system is present. Nerves are 

 accompanied by Schwann cells. Cartilage may appear in presence of the pharyngeal gut 

 and also of the stomachal region. 



