456 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



equipment capable of utilizing amino-acid molecules with the two phenyl- and 

 a-amino groups. On this basis, this supplementary metabolite is necessary and 

 sufficient to induce differentiation of neural crest by mobilizing this pre-existing 

 machinery. This fine analysis is now being extended with reference to the possible 

 activity of precursors of phenyl alanine (Wilde, ig56a). 



The second aspect of neural crest activities concerns the induction exerted by 

 it and is especially worth consideration, for it discloses a so far unnoticed modality 

 of induction. It has been long known that the mesenchyme of the dorsal fin formed 

 by amphibian tadpoles is a derivative of the neural crest, and is even the inductor 

 of this epiblastic, keel-shaped fold. In 1941, Twitty and Bodenstein already insisted 

 that in this complex the epiblast was not passive, but cooperated actively in the 

 modelling of the small organ. This idea has been worked out by Bodenstein (1952) 

 in a series of transplantation experiments which are really exemplary. His simple 

 but important conclusion is that, when the inductor is utilized to build op the 

 organ, which is not the rule for earlier inductions, the neural crest first influences 

 the epiblast to grow into a fold, and then creeps into the pocket formed. In addition, 

 by this very process, the epiblast acquires the capacity to influence the included 

 neural crest cells to adopt their special arrangement and become turgescent. The 

 same mechanism of interplay is valid for the formation of balancers, by assuming 

 some more general conditions inherent to the head region. This is indeed reciprocal 

 induction, a concept which can be used now in considering the mechanisms of the 

 formation of the paired limbs. 



{c) Limb buds 



A score of problems emerges when we try to understand the limbs of walking 

 vertebrates. How did these appendages evolve out of the paired fins of swimming 

 vertebrates? Why, in practically all vertebrates, does a so-called Wolffian crest 

 of mesenchyme form all along the flanks at a definite dorso-ventral level, near 

 the ventral boundary of the somites? Why do the major parts of these crests rapidly 

 disappear only to leave four small heaps of mesenchymatous cells, two anterior 

 and two posterior ones, adherent to the most dorsal part of the somatopleure? 

 Why do the regions of the trunk, surrounding each young bud, acquire and pre- 

 serve during the whole life of the vertebrate the potencies of a limb-forming field? 

 Why does the epiblast covering the young bud rapidly thicken into a dome- 

 shaped or acuminated apical cap? Why does the growing bud show a distal 

 paddle, evidently announcing the formation of a hand or foot? Which factors do 

 cause the earliest differentiations inside the mesenchymatous filling? Why does 

 the paddle, and soon also the narrowed parts, elongating in arm and fore arm, or 

 in thigh and leg, early acquire a characteristical asymmetry? Which conditions 

 do provoke the ordered bulging of the finger buds? How is the location and shape of 

 the cartilage rudiments, including the girdles, determined? How do the muscles 

 organize in such a characteristic fashion? How are other anatomical features, such 

 as joints, blood vessels, and nerve tracts, to be explained? Why are the details of 

 these processes different in the upper and lower limbs? What are the mechanisms 

 acting in regeneration of the formed limbs? How are their other so variable 

 morphological features to be explained? 



