FEATHERS AND PATTERNS
31
in the pulp apex depend on a variety of factors. The effective life ("viability"; Hamilton, 1940), barring, and the ability to produce pigment of
various colors (or to refrain altogether) are intrinsic to the melanocyte.
Other characteristics, for example, response to hormones and X irradiation, and the variation in pigmentary response around each feather collar,
are mediated by other tissues, of which the epidermis is a prime suspect.
Temporal as well as spatial parameters have their effect (see Figs. 2C
and 13). Trinkaus (1953) showed that, even though the feathers of the
adult breast tract of the Brown Leghorn responded to estrogen and
thyroxine by changes in pigmentary activity (estrogen produced rust
feathers and thyroxine produced black feathers), those of the same tract
of the embryo and young chicks did not. Furthermore, adult pigment
cell stock from this tract transplanted to "immature" feather germs failed
to respond. Trinkaus considers this finding to be evidence for the relatively late acquisition by the epidermis of the ability to mediate pigmentary responses. Dubious evidence of such an effect was also found by
Cohen (1963) for X-irradiation depigmentary effect. Trinkaus also
showed "maturity" to be acquired at different times by feather follicles
in different tracts; indeed, it could be hastened in a single follicle by
repeated plucking.
Another temporal effect concerns the lack of pigmentation at the bases
of most feathers (hairs and vibrissae also show this). No hypothesis
seems to have been advanced to cover this case. Some of the X-irradiated follicles described by 'Espinasse (1959) produced feathers with
much longer or shorter white regions; therefore, this phenomenon may
perhaps tentatively be ascribed to an extrinsic effect, rather than a
"using up" of pigment cells, or a barring-type phenomenon with wavelength equal to feather length. The work of Cleffmann (1960) on agouti
hairs is also very relevant in this connection.
The adoption by a melanocyte of a position in a barb ridge results in
a contiguity with several epidermal cells. This "epidermal melanin unit"
is probably more real than that in mammalian skin (Szabo, 1965) since
the melanocytes seem not to wander at all. Indeed, the crispness of the
pattern of most feathers would be precluded by such wandering as is
indulged in by the epidermal melanocytes of mammals. Furthermore two
contiguous melanocytes may be producing pigment of different colors
(Fig. 14B). That this reflects an epidermal discontinuity seems proven,
but, as yet, we have no clue to its nature.
VI. Repetition and Register
The repetition of transverse patterns in successive feathers has already
been mentioned. The attempt at explanation by Cohen and 'Espinasse
(1961) still seems the most satisfactory, but is not altogether convincing.
31
in the pulp apex depend on a variety of factors. The effective life ("viability"; Hamilton, 1940), barring, and the ability to produce pigment of
various colors (or to refrain altogether) are intrinsic to the melanocyte.
Other characteristics, for example, response to hormones and X irradiation, and the variation in pigmentary response around each feather collar,
are mediated by other tissues, of which the epidermis is a prime suspect.
Temporal as well as spatial parameters have their effect (see Figs. 2C
and 13). Trinkaus (1953) showed that, even though the feathers of the
adult breast tract of the Brown Leghorn responded to estrogen and
thyroxine by changes in pigmentary activity (estrogen produced rust
feathers and thyroxine produced black feathers), those of the same tract
of the embryo and young chicks did not. Furthermore, adult pigment
cell stock from this tract transplanted to "immature" feather germs failed
to respond. Trinkaus considers this finding to be evidence for the relatively late acquisition by the epidermis of the ability to mediate pigmentary responses. Dubious evidence of such an effect was also found by
Cohen (1963) for X-irradiation depigmentary effect. Trinkaus also
showed "maturity" to be acquired at different times by feather follicles
in different tracts; indeed, it could be hastened in a single follicle by
repeated plucking.
Another temporal effect concerns the lack of pigmentation at the bases
of most feathers (hairs and vibrissae also show this). No hypothesis
seems to have been advanced to cover this case. Some of the X-irradiated follicles described by 'Espinasse (1959) produced feathers with
much longer or shorter white regions; therefore, this phenomenon may
perhaps tentatively be ascribed to an extrinsic effect, rather than a
"using up" of pigment cells, or a barring-type phenomenon with wavelength equal to feather length. The work of Cleffmann (1960) on agouti
hairs is also very relevant in this connection.
The adoption by a melanocyte of a position in a barb ridge results in
a contiguity with several epidermal cells. This "epidermal melanin unit"
is probably more real than that in mammalian skin (Szabo, 1965) since
the melanocytes seem not to wander at all. Indeed, the crispness of the
pattern of most feathers would be precluded by such wandering as is
indulged in by the epidermal melanocytes of mammals. Furthermore two
contiguous melanocytes may be producing pigment of different colors
(Fig. 14B). That this reflects an epidermal discontinuity seems proven,
but, as yet, we have no clue to its nature.
VI. Repetition and Register
The repetition of transverse patterns in successive feathers has already
been mentioned. The attempt at explanation by Cohen and 'Espinasse
(1961) still seems the most satisfactory, but is not altogether convincing.