8
JACK COHEN
2. Barbs and Rachis
The barbs and the rachis form a system which seems at first sight to
provide ideal experimental material for a study of morphological pattern
development. Work in this field, however, was long delayed by the misleading theory of feather development proposed by Lillie (see, for example, Lillie, 1942; Lillie and Juhn, 1938; Lillie and Wang, 1941, 1943,
1944, or the criticism by 'Espinasse, 1939). The main proposal of this
theory was that tissue really moved tangentially around the feather
collar and produced barb ridge distally until it fused with the rachis.
This theory put barb development into the same category of pattern
formation as the production of a series of tapeworm proglottides or
extrusion through a moving die of special shape. Neither example presents
more than a historical or topological problem. The discovery that barbs
and barbules may arise within collar tissue in culture (Cohen, 1957),
however, made Lillie's view untenable and also brought their formation
into the realm of the experimentally accessible.
3.
Pigmentation
The pigmentation patterns of feathers form another ideal system for
pattern analysis. The melanoblast -» melanocyte system is exquisitely
sensitive to changes in tissue environment (see, for example, Markert and
Silvers, 1956; Willier, 1952; Reams, 1957) and has the advantage of
producing permanent records of experimental results. The system of
control is extraordinarily complex and is discussed at greater length
below.
l f . Patterns of
Keratinization
Apart from the somewhat gross keratinization patterns of the barbs and
barbules, subtle differences in extent and nature of keratinization account
for the iridescence associated with many feathers; the "eyes" on the
tail of the peacock are a conspicuous example, but the phenomenon is, in
fact, very common. Such "structural" color is seen in the blue budgerigar
and in many of the green and blue sheens on black birds.
D. The Keratin Itself
Keratin, as used by biologists, is usually not a chemical term. It refers
to the complex of debris (see Matoltsy, 1965) resulting from the mummification and drying out of certain epidermal cells; these cells are
specialized in that during this process a matrix of protein is constructed.
This may range in hardness from claw through to scurf, and may be
associated with crystals of calcium salts (e.g., in the claws of carni-
JACK COHEN
2. Barbs and Rachis
The barbs and the rachis form a system which seems at first sight to
provide ideal experimental material for a study of morphological pattern
development. Work in this field, however, was long delayed by the misleading theory of feather development proposed by Lillie (see, for example, Lillie, 1942; Lillie and Juhn, 1938; Lillie and Wang, 1941, 1943,
1944, or the criticism by 'Espinasse, 1939). The main proposal of this
theory was that tissue really moved tangentially around the feather
collar and produced barb ridge distally until it fused with the rachis.
This theory put barb development into the same category of pattern
formation as the production of a series of tapeworm proglottides or
extrusion through a moving die of special shape. Neither example presents
more than a historical or topological problem. The discovery that barbs
and barbules may arise within collar tissue in culture (Cohen, 1957),
however, made Lillie's view untenable and also brought their formation
into the realm of the experimentally accessible.
3.
Pigmentation
The pigmentation patterns of feathers form another ideal system for
pattern analysis. The melanoblast -» melanocyte system is exquisitely
sensitive to changes in tissue environment (see, for example, Markert and
Silvers, 1956; Willier, 1952; Reams, 1957) and has the advantage of
producing permanent records of experimental results. The system of
control is extraordinarily complex and is discussed at greater length
below.
l f . Patterns of
Keratinization
Apart from the somewhat gross keratinization patterns of the barbs and
barbules, subtle differences in extent and nature of keratinization account
for the iridescence associated with many feathers; the "eyes" on the
tail of the peacock are a conspicuous example, but the phenomenon is, in
fact, very common. Such "structural" color is seen in the blue budgerigar
and in many of the green and blue sheens on black birds.
D. The Keratin Itself
Keratin, as used by biologists, is usually not a chemical term. It refers
to the complex of debris (see Matoltsy, 1965) resulting from the mummification and drying out of certain epidermal cells; these cells are
specialized in that during this process a matrix of protein is constructed.
This may range in hardness from claw through to scurf, and may be
associated with crystals of calcium salts (e.g., in the claws of carni-