FEATHERS AND PATTERNS
29
organization of the pigment cell reservoir (see Cock and Cohen, 1958)
and that of the feather. A suggestion by Chase et al. (1963), in connection with X-irradiation depigmentation, that pigment cells occupy specific
"niches" in the epidermis, offers a mechanism by which this could be
accomplished. Any repetition (see Cohen and 'Espinasse, 1961) requires
a rigorous specificity of topological transformation between papillär ectoderm and collar and between collar and feather. If this ectoderm has
pigment cells in specific niches, then Cock's fleck repetition would also
be accounted for. Both pigmented and unpigmented flecks and patches
were observed by Cole and Finley (1941) after X-irradiation of pigeon
tailfeathers. These also showed repetition, and these workers argued for
a mutation explanation, or, alternatively, for a selective destruction
hypothesis such as that of Chase (1951). However, Cohen (1963)
showed that the effect of X-irradiation on depigmentation is not primarily upon the pigment cells, but is an indirect effect (see also Chase
et al., 1963). The epidermis seemed a likely candidate as mediator of the
X-ray effect. It is possible, therefore, that unlike Cock's pigmented
flecks and patches, unpigmented flecks or patches associated with trauma
or experimental injury (see Taylor, 1949) are due to changes in the
epidermis concerned, the pigment cell stock remaining potent. That
the epidermis may itself be under the influence of the dermis in this
regard is discussed further below.
C. Unpigmented Birds
These must be distinguished from the negligibly pigmented birds already dealt with in category 2, above; this category includes the true
albinos only. It is probably metaphysical to point out that these birds
do have latent feather pattern, that they do, indeed, have the "genetically
determined stencil" in their feather collar ectoderm, but that they do not
have competent pigment cells to reveal this cryptic organization. Nevertheless, the point is an interesting one.
V. Pigmentation of the Individual Feather
Here is a very interesting situation, indeed. Melanoblasts with specific
genetic potential enter feather germs from neighboring tissues (Danforth and Foster, 1929; Cock and Cohen, 1958) and perhaps enter
specific niches in the papillär ectoderm. Here they are (passively?)
carried into the collar, where they may become or produce melanocytes
(Fig. 14). The kind and amount of pigment produced, the effective life,
the turning on and off of this machinery seen in barring, the ability to
donate melanin, and the final fate of the melanocytes as debris in the
feather itself or as "senile" melanocytes (Hamilton, 1940; Cohen, 1957)
29
organization of the pigment cell reservoir (see Cock and Cohen, 1958)
and that of the feather. A suggestion by Chase et al. (1963), in connection with X-irradiation depigmentation, that pigment cells occupy specific
"niches" in the epidermis, offers a mechanism by which this could be
accomplished. Any repetition (see Cohen and 'Espinasse, 1961) requires
a rigorous specificity of topological transformation between papillär ectoderm and collar and between collar and feather. If this ectoderm has
pigment cells in specific niches, then Cock's fleck repetition would also
be accounted for. Both pigmented and unpigmented flecks and patches
were observed by Cole and Finley (1941) after X-irradiation of pigeon
tailfeathers. These also showed repetition, and these workers argued for
a mutation explanation, or, alternatively, for a selective destruction
hypothesis such as that of Chase (1951). However, Cohen (1963)
showed that the effect of X-irradiation on depigmentation is not primarily upon the pigment cells, but is an indirect effect (see also Chase
et al., 1963). The epidermis seemed a likely candidate as mediator of the
X-ray effect. It is possible, therefore, that unlike Cock's pigmented
flecks and patches, unpigmented flecks or patches associated with trauma
or experimental injury (see Taylor, 1949) are due to changes in the
epidermis concerned, the pigment cell stock remaining potent. That
the epidermis may itself be under the influence of the dermis in this
regard is discussed further below.
C. Unpigmented Birds
These must be distinguished from the negligibly pigmented birds already dealt with in category 2, above; this category includes the true
albinos only. It is probably metaphysical to point out that these birds
do have latent feather pattern, that they do, indeed, have the "genetically
determined stencil" in their feather collar ectoderm, but that they do not
have competent pigment cells to reveal this cryptic organization. Nevertheless, the point is an interesting one.
V. Pigmentation of the Individual Feather
Here is a very interesting situation, indeed. Melanoblasts with specific
genetic potential enter feather germs from neighboring tissues (Danforth and Foster, 1929; Cock and Cohen, 1958) and perhaps enter
specific niches in the papillär ectoderm. Here they are (passively?)
carried into the collar, where they may become or produce melanocytes
(Fig. 14). The kind and amount of pigment produced, the effective life,
the turning on and off of this machinery seen in barring, the ability to
donate melanin, and the final fate of the melanocytes as debris in the
feather itself or as "senile" melanocytes (Hamilton, 1940; Cohen, 1957)