26
JACK COHEN
ideal example would show melanocytes in every tissue to which melanoblasts have penetrated. The genetically determined threshold of melanoblast response to local environmental stimulus is so low that it is
exceeded in virtually every situation. Because of the near totality of the
pigmentation, sexual differences are not apparent.
B. Partially Pigmented Birds
From the point of view of the plumage alone, these fall into three
categories :
1. Those breeds in which the plumage is totally pigmented (except
for the fluff and basal rachis and calamus of most feathers), but melanocytes are lacking in some or all the other tissues in which it is known
they may be found (see Fig. 12). The Brown Leghorn and Rhode Island
Red are examples of fowls with this pigmentation pattern, and many
wild birds (e.g., black birds) show a similar case. Although all the
feathers are pigmented, the pigment, as in the Brown Leghorn, may be
different in color in the feathers of different tracts or in different regions of the same feather (see Figs. 2A and 12). Sexual dimorphism is
very usual in this latter case, and not only estrogens (Trinkaus, 1953;
'Espinasse, 1939) but a wide variety of other hormones (Willier, 1948)
may modify the feather colors.
2. Those birds in which the plumage is white, but the ability to form
melanin has not been suppressed in other tissues. The White Silkie has
copious pigmentation of its peritoneum and liver, and of much of its
dermis, but few epidermal or feather collar melanocytes. The White
Leghorn has pigmented irises and, rarely, a pigmented omentum and
oviduct, but white plumage. The White Wyandotte has few melanocytes
in its integument or its internal organs, but does have melanin pigment
in the iris. Hamilton (1940) showed that in breeds such as these, melanocytes are produced in the feather collars, but donate melanin for so
short a period that the contribution of pigment to the feathers is negligible. He also showed that dominant white (IICC) birds such as the White
Leghorn produced more melanocytes than double récessives (iicc) such
as the Wyandotte. Cock (1956) has some comments on this genetic
situation.
Some white mammals, for example, the so-called "albino" guinea pig,
show a still more extreme case. Even the irises are unpigmented, but
chilled regions of the body (ears and feet) often show slight pigmentation of the dermis and epidermis. Whether this is to be regarded as a case
of partial pigmentation is a question for the philosophers. Some birds,
such as the swan and some geese, produce pigmented feathers when
juvenile but not when adult. Such temporal changes are referred to again
in Section VII.
JACK COHEN
ideal example would show melanocytes in every tissue to which melanoblasts have penetrated. The genetically determined threshold of melanoblast response to local environmental stimulus is so low that it is
exceeded in virtually every situation. Because of the near totality of the
pigmentation, sexual differences are not apparent.
B. Partially Pigmented Birds
From the point of view of the plumage alone, these fall into three
categories :
1. Those breeds in which the plumage is totally pigmented (except
for the fluff and basal rachis and calamus of most feathers), but melanocytes are lacking in some or all the other tissues in which it is known
they may be found (see Fig. 12). The Brown Leghorn and Rhode Island
Red are examples of fowls with this pigmentation pattern, and many
wild birds (e.g., black birds) show a similar case. Although all the
feathers are pigmented, the pigment, as in the Brown Leghorn, may be
different in color in the feathers of different tracts or in different regions of the same feather (see Figs. 2A and 12). Sexual dimorphism is
very usual in this latter case, and not only estrogens (Trinkaus, 1953;
'Espinasse, 1939) but a wide variety of other hormones (Willier, 1948)
may modify the feather colors.
2. Those birds in which the plumage is white, but the ability to form
melanin has not been suppressed in other tissues. The White Silkie has
copious pigmentation of its peritoneum and liver, and of much of its
dermis, but few epidermal or feather collar melanocytes. The White
Leghorn has pigmented irises and, rarely, a pigmented omentum and
oviduct, but white plumage. The White Wyandotte has few melanocytes
in its integument or its internal organs, but does have melanin pigment
in the iris. Hamilton (1940) showed that in breeds such as these, melanocytes are produced in the feather collars, but donate melanin for so
short a period that the contribution of pigment to the feathers is negligible. He also showed that dominant white (IICC) birds such as the White
Leghorn produced more melanocytes than double récessives (iicc) such
as the Wyandotte. Cock (1956) has some comments on this genetic
situation.
Some white mammals, for example, the so-called "albino" guinea pig,
show a still more extreme case. Even the irises are unpigmented, but
chilled regions of the body (ears and feet) often show slight pigmentation of the dermis and epidermis. Whether this is to be regarded as a case
of partial pigmentation is a question for the philosophers. Some birds,
such as the swan and some geese, produce pigmented feathers when
juvenile but not when adult. Such temporal changes are referred to again
in Section VII.