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Re: Re.: If Dinosaurs Could Fly and an old 'axiom'



>To the developmental biologists out there: what does a feather look like
>during "plumagogenesis" or whatever it would be called (feathery equivalent
>to odontogenesis)?  Do feathers begin structurally during development as a
>spike which sends out shoots, which themselves send out shoots, etc.?  Do
>feathers begin developmentally as a flat plate with a central shaft, with
>material removed from the plate to form the feather substructures?
>Inquiring, bone-biased minds want to know!
>
>Thomas R. Holtz, Jr.

The following is taken from the article "feather" in "A Dictionary of Birds":

"At about the fifth day of incubation the embryo of a fowl may be seen to
have rows of pimples, in well-marked patterns, on its surface... there is a
finger of dermis pushed out, covered by a thimble of epidermis... This is a
feather germ.

"Further localized growth now serves to push this feather germ down into a
deepening depression.  While this is happening, the feather germ continues
to lengthen greatly, so that, instead of a pimple on the surface of the
embryo, there is now a pit with a long cylinder projecting out of the mouth
of a follicle.  The cylinder slopes backwards, so that obverse and reverse
sides can be distinguished.

".....The cells forming the outside of the [epidermal] cylinder, when they
keratinize, are joined together to constitute a resistant sheath.  The
sheath is destined later to split and be lost....

"The inner aspect of the lengthening cylinder becomes longitudinally
pleated or ridged.  These ridges contain the developing barbs.  Within the
substance of the barb-ridges many cells fall away individually and take no
part in the structure of the finished feather, but some become firmly
joined to each other and form the rami of the barbs.  Other cells become
joined in columns, each column sloping downwards towards its barb, to which
proximally it becomes firmly fixed.  Two series of such columns are
differentiated in each barb-ridge, and these columns of single cells will
be the barbules when they have become thoroughly keratinized.

"In the developing contour feather the barb-ridges each take a half-spiral
course... The two fail to meet dorsally, and the strip down the cylinder
dorsally thus left intact is the site of the developing rachis."
--
Ronald I. Orenstein                           Phone: (905) 820-7886
International Wildlife Coalition              Fax/Modem: (905) 569-0116
1825 Shady Creek Court                 
Mississauga, Ontario, Canada L5L 3W2          mailto:ornstn@inforamp.net