460 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



formed by the association of dermal mesenchyme with epiderm, hke the Hmb bud. 

 The size of each complexed unit is indeed much more reduced, but it is repeated 

 many times, and the feather germ tracts are disposed according to a distinct 

 pattern, the determinism of which has been explored by Saunders and Gasseling 

 (1957). The possibilities offered by trypsin dissociation and in vitro cultivation 

 have enabled Sengel (1958) to analyse the mechanisms at play in these processes. 

 The method consists in combining epiderm and mesenchyme of different ages and 

 origins. It reveals a succession o^ four phases. 



^\\e first one is of a banal character and concerns the induction of ectoblast in epiblast 

 under the influence of the peripheral mesenchyme. In a second phase, this mesenchyme, which 

 is thus a mixture of mesoblast and ectomesoblast, becomes arranged into distinct mounds, 

 which may be presumed heterogeneous. In 7-day embryos, a medio-dorsal crest appears 

 along the spine, as far as the root of the tail, and soon becomes fragmented in 22 small masses 

 of mesenchyme cells (Holmes, 1935) which are as many germs of feathers appearing as 

 low skin elevations. During the end of the same day and the 8th, new mounds appear 

 successively, in dorso-ventral direction, along parallel rows, but disposed in quincunx and 

 perfectly equidistant. This regularity of pattern suggests the competitive utilization of 

 some trophic material. When Sengel isolated at 6 3/4 days a rectangular piece of back skin 

 cut on one side of the spine and already endowed with three median rows of papillae, he 

 observed in culture the regression of these germs and, after one to two days, the reappearance 

 of a new row along the median line of the fragment and the gradual differentiation of other 

 parallel rows. Thus, the pattern of feather germ is apt to complete regulation under new 

 trophic conditions. Consequently, the acquisition of the normal pattern depends on a 

 general factor present under the skin, and I should prefer, contrary to the author, not to 

 consider this second phase as an induction. By contrast, the third phase is definitely inductive. 

 The mesenchymal condensations cause the epiderm to bulge in buds which will be deter- 

 mined as feathers or scales according to the nature of the mesenchyme, the latter differentia- 

 tion being specific of the tarso-metatarsal region. During the fourth phase the epiderm exerts 

 a way-back induction which has a double role : it fixes the direction of the feathers or scales 

 according to the cephalo-caudal polarity of the epiderm and it causes the dermal cells to 

 colonise the epidermal sheath of the feathers. 



Compared to the formation of limb bud, the acquisition of phanera — the above 

 analysis may be presumed to be largely valid for hairs — shows a general simi- 

 larity of mechanisms, but the way-back induction plays a minor role, and only in 

 the terminal phase of the process. This difTerence relates evidently to the epidermal 

 differentiation of the apical region in the limb bud. 



In the formation of feathers, the cytochemical aspects observable in the young 

 buds are remarkable. The dermal concentrations of mesenchyme are rich in both 

 RNA and in alkaline phosphatase. When the shaft has grown upward, an interest- 

 ing dissociation appears. The parts richer in RNA are in the epiderm, and their 

 variations are paralleled in the mesenchyme by the alkaline phosphatase (Koning 

 and Hamilton, 1954). It seems that the metabolite and the enzyme cooperate 

 intimately. Cultures of embryonic skin fragments have recently been grown, in 

 conditions which permit the introduction of the Beryllum basic acetate [Be04 

 (CjHjOa)^], which is considered to be a specific inhibitor of the phosphatases 

 (Hamilton and Koning, 1956). The effects are quite striking because, at a given 

 level of inhibition, the feathers resemble hairs. This is indeed very suggestive of 

 the physiological importance of these biochemical factors. Of course, every biologist 

 is conscious that these are only two entities among many molecular populations 



