Ill SECONDARY AND MINOR INDUCTIONS 46 1 



which are operative in the process. Nevertheless, the association of RNA and 

 alkaline phosphatase (s) , eventually combined with mucopolysaccharides and acidic 

 phosphatase(s) too frequently happens to be deprived of significance. To quote these 

 same authors: "induction or morphogenetic activity at any level of organization 

 involves a chain of reactions, with alkaline phosphatase providing molecular units 

 for the synthesis of RNA and the latter mediating the synthesis of proteins and 

 morphogenesis proper"' (1956, p. 70). 



Similar remarks could be presented concerning the cytochemistry of the hair 

 buds, which has just been accurately studied by Achten (1959) in human and rat 

 embryos. 



The iiropygial gland of the duck recently has been submitted to a parallel in- 

 vestigation, both in vivo and in vitro (Lutz and Gomot, 1956; Gomot, 1956a, b). 

 The folding of the epiblast in the sacral region of the caudal bud is preceded by 

 the appearance of alkaline phosphatase and RNA in a subjacent layer of mesen- 

 chyme. When the epiblastic furrows produce buds, the same indices of over- 

 activity are found in their cells. Trypsic dissociations of the caudal bud and culture 

 of its components, isolated or combined, have shown that the inductor is the outer 

 layer of the mesenchyme, only limited by the phosphatase line. This layer of 

 mesenchyme is endowed with the special glandulogenic activity, which is absent 

 either in pectoral or in feather buds mesenchyme. The epiblast, as ordinarily, is 

 unspecialized; it may be transplanted from any part of the body, and even be 

 taken from chicken or rabbit. With toad, frog or trout epiblast, first steps of 

 uropygial differentiation are obtainable (Gomot, 1958). 



{e) The induction of cartilages and somites 



As already mentioned, the cartilages encapsulating the sense organs (p. 453) 

 and those sustaining the branchial arches (p. 454) are known to be induced. The 

 same is true for the vertebral arches, as shown by operations performed by Wat- 

 terson (1952) on Fnndulus embryos, by H. Holtzer and Detwiler (1953) and by 

 Detwiler (ig54a) on salamander embryos, and by Strudel (1955) on the chick. 



In the case of amphibians^ at the tail bud stage, removal of the spinal cord and 

 of parts of the somites and the grafting of the neural tube, with or without noto- 

 chord, in the somite region, demonstrate that the neuraxis acts by establishing 

 somite- growth centers and inducing the cartilages. 



The same mechanism works also in chick embryos. According to Lash and Holtzer 

 (1957 a, b) the inductive influence of the spinal chord passes through a millipore membrane; 

 the material transmitted is not a pure fluid but contains Bodian positive particles. Notochord 

 does not play any role for chondrogenesis in amphibians, but in birds, this organ, although 

 not indispensable for somite genesis (Fraser, 1958), is necessary to obtain the vertebral 

 bodies. The chondrogenic influence of the chick notochord has been confirmed by the 

 investigations of Avery, Chow and Holtzer (1956). It manifests itself when somites of appro- 

 priate stages (14 to 16) are cultured along with notochord. Contrary to what happens 

 when using spinal cord, the cartilage appears in contact with the inductor. According to 

 Strudel, the spinal cord not only exerts a chondrogenic role, but also controls the produc- 



^ See also Muchmore (1958). 



^ If somatopleural material of frog embryos is grafted in contact with the hind-brain, muscle 



cells are obtained (Liedke, 1958). 



Literature p. 483 



