458 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



used for: (a) performing 180" and 90" rotations of the epiblast on the mesenchyme mound 

 at the third day (ZwilHng 1956a) ; (b) covering a part of this mesenchyme with an apical 

 sHver, or applying two or even three apical slivers on one mesenchyme mound (Zwilling, 

 1956b); (c) exchanging the epiblast and mesenchyme between buds of normal and poly- 

 dactylous strains (Zwilling 1956c); and (d) effecting the same exchange between normal 

 and wingless strains. The results can be interpreted along the lines traced just above, but 

 naturally add some details to the general interpretation. The reality of an interaction 

 between apical ridge and mesenchyme appears from the fact that in expt. a, the 180° 

 rotation gives an asymmetry conforming to the mesenchyme, while a 90° rotation results 

 in an asymmetry conforming to the epiblast. This difference is interpreted as a function 

 of the deeper perturbation of the system in the second case, so that the tendency already 

 inherent in the epiblast can express itself. The existence of an influence exerted by the 

 apical ridge is also made apparent by the fact that in expt. b, a considerable reduction of 

 the mesenchyme mass allows the formation of a normal limb, while covering a whole 

 mesenchyme mound with two apical ridges causes reduplication; however, grafting three 

 apical ridges in a tandem-like fashion does not change the limb unity, which shows that 

 the lateral parts of the mesenchyme are not as reactive as the distal part. This influence 

 is expressed by Zwilling as "growth inducing factor" and the first indications of this action 

 can be noticed, in the rat limb bud, by the peripheral accumulations of RNA, the ap- 

 pearance of the peripheral veinous sinus, and the synthesis of glycogen around this vessel. 

 Finally, expt. c and d show that the mesenchyme continues to exert an action on the apical 

 epiblast even after the 3rd day of development. This is shown by the fact that if a normal 

 apical sliver is grafted on the mesenchyme mound of a polydactylous limb^, this apical 

 ridge will extend more than usual on the pre-axial side, and its growth-promoting action 

 will thereby cause Polydactyly. Reciprocally, when a normal epiblast is combined with 

 "wingless" mesenchyme, the apical ridge begins to form, but soon recedes and the growth 

 of the bud stops. 



These delicate interrelations between mesenchyme and apical epiblast of the limb bud 

 have been submitted to further investigation. Amprino and Camosso (1958a, b) have 

 used once more the insertion of Carbon particles for a precise mapping of the presumptive 

 territories. There is no doubt that the distal parts of the mesenchyme arise progressively 

 by the expansion of a tiny matrix formed of a few cells, and that, on the way, the topogra- 

 phical relations of the mesenchyme with the apical ridge change gradually. By experiments 

 of section, rotation, reimplantation, these authors have obtained reduplications which cannot 

 be explained by a perturbation of the interactions between mesenchyme and apical ridge; 

 they reveal another influence exerted by the mesenchyme present at the basis of the bud. 



Saunders, Cairns and Gasseling (1957) have transplanted mesenchyme of the thigh bud 

 (stage 20) into the wing bud (stage 21) in various locations, realizing different relations of 

 the graft with the apical ridge. From the nature of appendices and feathers obtained, "it is 

 concluded that the apical ridge exercises an inductive action on the subjacent limb 

 mesoderm, bringing about the formation of terminal limb parts, and that this influence 

 affects the regional character of morphological differentiation and inductive specificity 

 of the mesoderm with respect to the proximo-distal axis of the limb but does not affect its 

 specific wing or leg quality". 



Thus, it remains admitted that this wing or leg quality is inherent to the mesen- 

 chyme and concerns the territory of the body (Guyenot, 1927) in which this 

 mesenchyme has been formed. Taking advantage of trypsin dissociations, 

 Saunders, Gasseling and David (1958) have methodically explored the conse- 

 quences of rotating the apical cap and recognized a predominant role of its 

 caudal region. By a remarkable correlation, this is also the territory of the cap 

 which is, at least in the bud of the fore limb of the rat, the richest in alkaline 

 phosphatase (Fig. 38, p. 365). 



^ The mound of a limb which will become polydactylous is not increased in size. 



