508 



Special Vertebrate Organogenesis 



and lens, many other ectodermal organs, 

 such as gills, balancers, fins, teeth, hypoph- 

 j^sis, etc., are dependent upon influences 

 exerted by other tissues for their differen- 

 tiation. Inductive influences, as Weiss ('35) 

 has pointed ovit, were originally thought of 



./^^ 



strating embryonic induction are extirpation 

 and transplantation. To a certain extent both 

 have been applied towards testing the in- 

 ductive faculties of the tissues comprising 

 the feather germs. Before considering the 

 experimental evidence, a few facts regarding 



em 



m 



em 



dp 



Fig. 194. Sections through the skin of saddle region of a normal Lakenvelder pullet. 3 weeks after hatching, 

 showing the structure of the fully differentiated skin and definitive feathers developing in situ. Delafield's 

 hematoxylin, 10/u. A, X 75; B, X 100. b. Cells of the barb system; c, collar (thick ring of embryonic feather- 

 forming cells); d, dermis or corium; dp, dermal papilla of feather; e, epidermis; em, erector muscles of 

 feather; /, follicle cavity; m, striated muscles; p, pulp of feather; r, rachis. 



as being much more specific than they are 

 considered to be at the present time. The 

 term induction has now been extended to 

 include a variety of "organizing" influences 

 from unspecific activation to the very specific 

 organization of typical patterns in space and 

 time. Inductive phenomena are by no means 

 physiologically uniform. 



The Feather. The two classic methods that 

 have been employed in general for demon- 



the origin of feather germs should be re- 

 called. The first indication of the site of 

 a prospective feather is seen in the meso- 

 dermal portion of the dermis or corium, at 

 approximately the fifth day of incubation, 

 in the form of an aggregation or condensa- 

 tion of cells immediately beneath the thin, 

 two-layered epidermis. This condensation is 

 the primordium of the dermal papilla and 

 precedes any visible epidermal response, as 



