SECONDARY AND MINOR INDUCTIONS 



457 



Most of these questions have still only a programmatic value, while two or 

 three of them have been extensively investigated. The analysis of asymmetry by 

 Harrison {cf. 1945) and his students has been one of the great embryological ac- 

 complishments of the interwar period, while the recent post-war years have 

 brought the partial elucidation of induction processes in the liml:) bud formation. 



The greater part of this work has been done on the chick embryo, first by Ham- 

 burger (1938) who opened the experimental way by grafting the limb buds, and 

 later by Saunders and by Z willing. More recently, descriptions of the cytochemical 

 changes in the rat and mouse limb-buds have added valuable information. The 

 basis of the general interpretation recently formulated is a mechanism of reciprocal 

 induction. The existence of the initial mesenchyme heaps, in their four sites, 

 being assumed as a starting point, these cell concentrations induce the covering 



Fig. 94. Role of apical ridge in the formation of the limb bud. Left, section of a normal 



4-day chick-embryo, with the wing bud covered by the typical, acuminated epiblast cap. 



Right, a wingless embryo of the same age. After Zwilling, 1949. 



epiblast, in which the alkaline phosphatase synthesis (Fig. 38, p. 365) charac- 

 teristic of induced epithelium can soon be detected. Then the leading role passes 

 to the apical ridge, which influences the mesenchyme, orients its first cytochemical 

 steps toward differentiation, and promotes its growth. (Milaire, 1956; Hin- 

 richsen, 1955). The installment of the blood vessels and their local effect on cell 

 nutrition (Milaire, 1956) combines its effects with the slight diflferences induced 

 in the mesenchyme to promote effective differentiation. 



The delicate role played by the apical ridge has been demonstrated by Saunders 

 (1948) and by Saunders et al. (1957, 1958) through methodical experiments per- 

 formed on the chicken embryo. The non-diflferentiation of an apical ridge in 

 embryos of a strain aflfected with a wingless syndrome (ZwilUng, 1949) confirms 

 (Fig. 94) the role attributed to this structure. 



A further advance has been made possible thanks to Zwilling's method (1955) of inter- 

 changing the epiblast and mesoblast of the limb buds. This skillful technique has been 



Literature p. 483 



