Ill SECONDARY AND MINOR INDUCTIONS 459 



This conception of limb development is not only valid for amniots. Tschumi (1956) has 

 extended it to Amphibians by clever experiments performed on Xenopus tadpoles. The 

 general interactions are of the same type, with the complement that the peripheral vessel is 

 induced by the apical cap, exactly as it has appeared to me from the cytochemical aspects 

 obtained by Milaire in the rat. 



In the realm of experiments on amphibians, one of the recent interesting results has 

 been the production of supernummerary anterior limbs in the neck when the whole 

 archenteric roof of this region had been replaced by epiblast. The mechanism of the anomaly 

 has now been elucidated (Fautrez, 1956). It depends on the sub-division of the limb bud 

 and on the migration of its dorsal half into the region where notochord and rhombencephalon 

 have been suppressed. How this local disturbance causes the doubling of the bud remains 

 to be explained^. 



Generally speaking, the problem of limb formation is certainly approaching a 

 solution valid for all tetrapods. The mesenchyme provides the epiblast with some 

 agent which, according to data on rodents, produces in this layer a limited RNA 

 synthesis, and a more evident production of cytoplasmic proteins, as well as 

 alkaline phosphatase(s). The mutants considered in the chick interfere with the 

 quantity of this agent secreted by the mesenchyme. Polydactylous strains form 

 too much of it, wingless strains too little. It is also interesting that the asymmetry 

 of a normal limb can be related to an unequal production of this agent in the 

 mesenchyme, at first probably distributed along a decreasing cephalo-caudal 

 gradient, and later also along a dorso-ventral one. 



Finally, let us mention that induction in the limb is not only a striking example 

 of reciprocal induction, but also represents, in the complex picture of indviction 

 processes, an uncommon mode: the apical ridge regresses as soon as the first steps 

 of morphochoresis are over and simply has the value of a transitory organ with 

 a necessary role in a phase of embryonic life. 



{d) Some epiblast derivatives 



Our rapid survey must be limited to two examples of present interest, teeth and 

 feathers. 



Teeth primarily proceed from mesenchyme mounds in the location of which 

 the regional endoblast and perhaps the branchial cartilages play a role. The 

 neural crest derivates included in these cartilages and probably also in the palate 

 could explain such inductive activities. This reaction of the epithelium covering 

 such primordia is probably also of inductive nature. As soon as the clock-shaped 

 rudiment begins to form, RNA becomes abundant in the cylindrical epiblast as 

 well as in the peripheral cells, which are the future odontoblasts of the mesen- 

 chyme mound. The intensity of basophily at successive stages, and the variations 

 in its pattern (Dalcq, 1953b; Hermann, 1956) again suggest a role of the ribo- 

 nucleoproteins whose activity is rapidly followed by the synthesis of alkaline 

 phosphatase (Dalcq and Mulnard, 1953). 



Feathers and scales, the two kinds of phanera present in birds, are evidently 



^ For other work too specialized to be considered here, cf. for example, Balinsky, 1934; 

 Perri, 1952; Dei, 1954; Holtfreter, 1955a. In regeneration, the necessary presence of nerve 

 fibers {cf. Butler and Schotte, 1949) may amount to an induction. The role of the field 

 surrounding the limb has been especially studied in Guyenot's laboratory {cf. Kiortsis, 

 1953). For the cytochemistry of regeneration, cf. Brachet, 1946a. 



Literature p. 



