Skin and Its Derivatives 



507 



space order to asymmetry in the adult plum- 

 age of the Brown Leghorn has been clearly 

 shown by Juhn and Fraps ('34a, b). These 

 investigators found that the symmetry re- 

 lations along transverse rows of feathers of 

 the breast tracts follow an orderly distribu- 

 tion with respect to a secondary axis lying 

 approximately at the sixth row, counting 



when the characteristics of the humeral tract 

 are becoming established has come from 180 

 degree rotation of small areas of ectoderm 

 and underlying mesoderm of the dorsal sur- 

 face of the wing buds of 3- to 4-day chick 

 embryos (Saunders, '50). While the resulting 

 wing plumage was normal in some cases, 

 showing complete regulation of the area, 



.-.ltd 



Fig. 193. Sections showing stages in development of clown feather. Iron hematoxylin. S/jl. A, Early stage 

 in feather formation, breast tract, 9-day embryo. Note aggregation of mesodermal cells (primordium of the 

 dermal papilla) beneath two-layered ectoderm (epidermis). X 157. B, Later developmental stage from same 

 feather tract. Note increased proliferation of mesodermal cells causing ectoderm to protrude beyond the 

 skin level, forming the characteristic feather buds. X 157. C, Feather buds of breast tract of 11-day embryo. 

 Note increase in length of feather bud and the thickening of ectoderm to form barb-ridges. X 75. 



laterally from the mid-line of the bird. The 

 increasing degree of asymmetry with refer- 

 ence to this secondary axis is in accord with 

 observations that pigmentation patterns, nor- 

 mal or hormone-induced, limited to one 

 vane-half of feathers of the breast tract show 

 the relation of mirror images within each 

 tract. Furthermore, the degree of asymmetry 

 in such feather patterns increases as the lat- 

 eral margins of the tract are approached. 

 Thus the degree of asymmetry of the advilt 

 plumage is foimd to increase with distance 

 from row number 6 (the first row to arise) 

 and to correspond with the time of origin 

 of the various rows of feather papillae com- 

 posing the tract. 



Some indication of the time in ontogeny 



there were a sufficient number of cases in 

 which abnormalities occurred in the distri- 

 bution and orientation of the feathers to 

 indicate that the tract characteristics of the 

 rotated area were partially established. 



Role of the Mesoderm in Epidermal Spe- 

 cialization. It would be of considerable in- 

 terest to know whether the highly specialized 

 epidermal outgrowths, feathers and hairs, 

 arise in situ through some localizing factor 

 in the ectoderm itself or whether they are 

 produced by reaction with the underlying 

 mesodermal tissue mass. In other words, are 

 these specialized skin derivatives the prod- 

 ucts of embryonic induction? Experimental 

 embryology has demonstrated that, in addi- 

 tion to the central nervous system, inner ear 



