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



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

What's their source? 

>"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.

So the feather is primarily a dermal, not epidermal, structure?  Makes sense.

>"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....

Sheaths are retained on the pygostyle (tail), no?  Sounds like a double
cylinder might be the primitive condition.

>"The inner aspect of the lengthening cylinder becomes longitudinally
>pleated or ridged.  

Let me be the first to say that I'm not clear what this means.  "Inner
aspect" meaning the developing rachis as opposed to the sheath, or the
reverse side of the rachis?  "Longitudinal" meaning the ridges run
length-wise along the rachis, or circumferentially around it? 

>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.  

I apologize for being thick, but I don't understand this either.  Does this
mean that the ridges cells form the barbs before the ramus develops to any
extent, attach to the ramus and sort of unfold from the ridges as the ramus
grows outward?

Several helpful people have tried, with only marginal success, to improve my
feather terminology.  If I'm still getting it wrong, please let me know.

  --Toby White