32
JACK COHEN
Barring is a strange case of this repetition. To ascribe it to an intrinsic
property of the melanocyte does not begin to face up to the problem.
The melanocytes do produce barring in feathers of different breeds
(Willier and Rawles, 1948), but the barring in different feathers of the
same bird may have different wavelength. Since the bars are produced
in sequence (unlike barb ridges) there must be rhythmic phenomena in
the feather collar. The period would vary from 1 to 20 days in the
various feathers of a Barred Plymouth Rock cockerel. Since Cock has
shown an action of the barring on the morphology of the feather (the
barbuleless edge), it is not unreasonable to suppose a coupling between
the two systems of the kind which Goodwin (1963) suggested as a possible basis for such phenomena. Both systems—cell division and growth in
collar cells, and pigment production by the melanocytes—would each
have intrinsic oscillation. (Because the chemistry of any cellular control
system has time delays that are disparate in sequential parts of the
system, nonlinear oscillation results.) Circadian rhythms of growth are
well known in epidermal systems (see review by Bullough and Laurence,
1964) and may be controlled or synchronized by adrenaline levels. Should
there be coupling (e.g., substrate pool competition or reciprocal endproduct inhibition) between this and the activity of the pigment cells in
melanin synthesis, then longer period oscillation of greater amplitude
could be expected; this would lead to cutoff of the pigment cell system
and barring. Such a hypothesis could be checked in a variety of ways
(see Goodwin, 1963). Indeed, should early results show promise, pigment
cells of the barred breeds could be ideal test material for Goodwin's
ideas, in vitro and in vivo.
An idea of the accuracy of control of the barring phenomenon can be
gained from Fig. 13, in which the barred primaries may be seen to have
very good register with one another. Not only wavelength is critical in
the production of such register; follicle spacing, feather angles, and the
time of initiation of barring all must be precisely controlled. Yet despite
this, examples of excellent register abound. The white ring on the neck
of the male mallard is a case in point; another is the lining up of the
spots and dashes on young moorhens. Cases of this kind occur in mammals, too. The bands on a crisp tabby cat, especially on the tail, and
(probably) the spots of the leopard (but not the cheetah) show register
between hairs whose dark region is at different levels. Short of a "genetically determined stencil" about 1 cm outside the epidermis, imagination
fails to supply a single hypotheses to account for this !
VII. Discussion and Conclusions
The fact that the tract distinctions of birds arise in the skin by nonspecific local, probably mechanical, influences of subdermal organs and
JACK COHEN
Barring is a strange case of this repetition. To ascribe it to an intrinsic
property of the melanocyte does not begin to face up to the problem.
The melanocytes do produce barring in feathers of different breeds
(Willier and Rawles, 1948), but the barring in different feathers of the
same bird may have different wavelength. Since the bars are produced
in sequence (unlike barb ridges) there must be rhythmic phenomena in
the feather collar. The period would vary from 1 to 20 days in the
various feathers of a Barred Plymouth Rock cockerel. Since Cock has
shown an action of the barring on the morphology of the feather (the
barbuleless edge), it is not unreasonable to suppose a coupling between
the two systems of the kind which Goodwin (1963) suggested as a possible basis for such phenomena. Both systems—cell division and growth in
collar cells, and pigment production by the melanocytes—would each
have intrinsic oscillation. (Because the chemistry of any cellular control
system has time delays that are disparate in sequential parts of the
system, nonlinear oscillation results.) Circadian rhythms of growth are
well known in epidermal systems (see review by Bullough and Laurence,
1964) and may be controlled or synchronized by adrenaline levels. Should
there be coupling (e.g., substrate pool competition or reciprocal endproduct inhibition) between this and the activity of the pigment cells in
melanin synthesis, then longer period oscillation of greater amplitude
could be expected; this would lead to cutoff of the pigment cell system
and barring. Such a hypothesis could be checked in a variety of ways
(see Goodwin, 1963). Indeed, should early results show promise, pigment
cells of the barred breeds could be ideal test material for Goodwin's
ideas, in vitro and in vivo.
An idea of the accuracy of control of the barring phenomenon can be
gained from Fig. 13, in which the barred primaries may be seen to have
very good register with one another. Not only wavelength is critical in
the production of such register; follicle spacing, feather angles, and the
time of initiation of barring all must be precisely controlled. Yet despite
this, examples of excellent register abound. The white ring on the neck
of the male mallard is a case in point; another is the lining up of the
spots and dashes on young moorhens. Cases of this kind occur in mammals, too. The bands on a crisp tabby cat, especially on the tail, and
(probably) the spots of the leopard (but not the cheetah) show register
between hairs whose dark region is at different levels. Short of a "genetically determined stencil" about 1 cm outside the epidermis, imagination
fails to supply a single hypotheses to account for this !
VII. Discussion and Conclusions
The fact that the tract distinctions of birds arise in the skin by nonspecific local, probably mechanical, influences of subdermal organs and