FEATHERS AND PATTERNS
33
that these distinctions are maintained in the adult by persistent epigenetic action of specific dermis on labile epidermis, this epidermis in
turn influencing the differentiation of pigment cells lying in it, is a
reasonable view consistent with the work described here. The nature of
the changes in the mesoderm which make it, but not the dermal papillae
induced in it, location-specific and the nature of the dermal control over
epidermal differentiation are fruitful paths for future endeavor.
It now seems most probable that patterns of follicle arrangement will
prove to be dependent upon systems such as those proposed by Turing,
but certainly in more sophisticated form for individual cases. Theories
involving "zones of inhibition," gradients, fields, and so on, so soon become epicyclic that their utility is very doubtful. On the other hand,
systems of this kind may be so cybernetically complex that we may have
to be content with this kind of approximation for a long while.
The nature of the change in the papilla, whose symptom is the change
from production of down feather to production of juvenile or contour
feather, is quite unknown. The ease of operation on the papilla, together
with its reconstitutive ability and apparently simple form, make this
situation an attractive one to investigate.
It has previously been suggested (Cohen and 'Espinasse, 1961) that the
collar becomes the feather. This accounts for a succession of feathers
from one follicle having the same length ; moreover, detail of the mode of
formation of collars from papillae seems to determine feather shape and
may account for the after-feather (Fig. 8). On the other hand, the nature
of the control of vane width (by slight variation of barb-ridge angle?)
and of asymmetry in the vane is still completely unknown.
A classification of pigmentary loci such as in Fig. 12 does bring some
simple order into a complex situation. However, there has been no
suggestion as to the nature of the controls exerted by other tissues on
pigment cells (beyond a pious invocation of Jacob and Monod), nor any
as to the effects of the genetic differences in pigment cells which change
their (multiple?) thresholds to such influences. The fact that such a gross
insult as X-irradiation destroys the ability of epidermal cells to "turn
on" melanoblasts, while leaving unimpaired their ability to make
feathers, makes it especially difficult to suggest mechanisms.
The barring phenomenon is an odd one. An explanation of similar
sophistication to Goodwin's hypotheses would be most welcome. Naïve
views of "inherent" barring do not help much, nor do naïve views of
"systemic rhythm" affecting the cells concerned. A rhythmic phenomenon
in hair growth, "crimp" in wool, has recently been the object of an
ingenious but simple theory. Chapman (1965) believes that crimp is due
to rhythmic activity of the arrector pili muscle network, which swings the
follicles to and fro and bends the keratogenous zone to either side with
33
that these distinctions are maintained in the adult by persistent epigenetic action of specific dermis on labile epidermis, this epidermis in
turn influencing the differentiation of pigment cells lying in it, is a
reasonable view consistent with the work described here. The nature of
the changes in the mesoderm which make it, but not the dermal papillae
induced in it, location-specific and the nature of the dermal control over
epidermal differentiation are fruitful paths for future endeavor.
It now seems most probable that patterns of follicle arrangement will
prove to be dependent upon systems such as those proposed by Turing,
but certainly in more sophisticated form for individual cases. Theories
involving "zones of inhibition," gradients, fields, and so on, so soon become epicyclic that their utility is very doubtful. On the other hand,
systems of this kind may be so cybernetically complex that we may have
to be content with this kind of approximation for a long while.
The nature of the change in the papilla, whose symptom is the change
from production of down feather to production of juvenile or contour
feather, is quite unknown. The ease of operation on the papilla, together
with its reconstitutive ability and apparently simple form, make this
situation an attractive one to investigate.
It has previously been suggested (Cohen and 'Espinasse, 1961) that the
collar becomes the feather. This accounts for a succession of feathers
from one follicle having the same length ; moreover, detail of the mode of
formation of collars from papillae seems to determine feather shape and
may account for the after-feather (Fig. 8). On the other hand, the nature
of the control of vane width (by slight variation of barb-ridge angle?)
and of asymmetry in the vane is still completely unknown.
A classification of pigmentary loci such as in Fig. 12 does bring some
simple order into a complex situation. However, there has been no
suggestion as to the nature of the controls exerted by other tissues on
pigment cells (beyond a pious invocation of Jacob and Monod), nor any
as to the effects of the genetic differences in pigment cells which change
their (multiple?) thresholds to such influences. The fact that such a gross
insult as X-irradiation destroys the ability of epidermal cells to "turn
on" melanoblasts, while leaving unimpaired their ability to make
feathers, makes it especially difficult to suggest mechanisms.
The barring phenomenon is an odd one. An explanation of similar
sophistication to Goodwin's hypotheses would be most welcome. Naïve
views of "inherent" barring do not help much, nor do naïve views of
"systemic rhythm" affecting the cells concerned. A rhythmic phenomenon
in hair growth, "crimp" in wool, has recently been the object of an
ingenious but simple theory. Chapman (1965) believes that crimp is due
to rhythmic activity of the arrector pili muscle network, which swings the
follicles to and fro and bends the keratogenous zone to either side with