10
JACK COHEN
out and "unfurling" of the feather cylinder, hooks and scrolls (proximal
and distal barbules) come into precise alignment (Fig. 4). The shape of
every cell and the nature of its contacts with its neighbors (Fig. 4B)
determine the shape assumed by the final dry structure. Of paramount
importance, however, is the nature of the intracellular keratinization
process and its variation in contiguous cells. This pattern of cellular
keratinization is the lowest level of morphogenesis to be considered here.
Although little work has been done on feathers, Wilgram et al. (1965)
have pointed the way with a very elegant study of genetic error in
human skin. Perhaps the biochemist and the physical chemist are more
at home in these nether regions, where pattern must be determined
more or less directly by molecular dimensions. The work of Bell
et al. should be referred to in this context (see Bell, 1965; Bell et al.,
1964; Ben-Or and Bell, 1965).
III. Perspectives from Experiment
A. The Feather Tracts
The beautiful regularity of feather tracts and other regional specificities of epidermal appendages has tempted many embryologists into this
field.
The transplantation experiments of Rawles (see review, 1965) and of
Sengel (1964) show clearly that the dermis is the prime controller of
tract specificity in the embryo. Therefore, in seeking an explanation of
the pattern of the tracts over the surface of the bird, influences via the
mesenchyme would seem to be logical suspects (compare Cairns and
Saunders, 1954). Sengel (1964) has demonstrated such an influence by an
extract of nervous tissue in the development of the first middorsal pteryla,
whose position, he thought, determined the dorsal pterylar pattern and
therefore the tract margins. This point is discussed further in the light
of his subsequent results (see below).
It has long been believed (e.g., Lillie and Wang, 1943; Wang, 1943;
Cohen, 1961, 1964) that factors resulting in maintenance of tract specificity in the adult bird or mammal reside in the epidermis. The evidence
for this view now seems weak (see Cohen, 1965, for a full discussion),
and it seems that in the adult, as in the embryo, the dermis controls the
mode of differentiation of the epidermis, not only as regards its surface
character, but also as regards appendages. The dermal papillae, however,
are still supposed not to have tract-specific properties or influence (see
below).
The feathers in the tracts of various breeds of fowl, or, indeed, of
various species of bird, often do not seem very similar. Willier and
JACK COHEN
out and "unfurling" of the feather cylinder, hooks and scrolls (proximal
and distal barbules) come into precise alignment (Fig. 4). The shape of
every cell and the nature of its contacts with its neighbors (Fig. 4B)
determine the shape assumed by the final dry structure. Of paramount
importance, however, is the nature of the intracellular keratinization
process and its variation in contiguous cells. This pattern of cellular
keratinization is the lowest level of morphogenesis to be considered here.
Although little work has been done on feathers, Wilgram et al. (1965)
have pointed the way with a very elegant study of genetic error in
human skin. Perhaps the biochemist and the physical chemist are more
at home in these nether regions, where pattern must be determined
more or less directly by molecular dimensions. The work of Bell
et al. should be referred to in this context (see Bell, 1965; Bell et al.,
1964; Ben-Or and Bell, 1965).
III. Perspectives from Experiment
A. The Feather Tracts
The beautiful regularity of feather tracts and other regional specificities of epidermal appendages has tempted many embryologists into this
field.
The transplantation experiments of Rawles (see review, 1965) and of
Sengel (1964) show clearly that the dermis is the prime controller of
tract specificity in the embryo. Therefore, in seeking an explanation of
the pattern of the tracts over the surface of the bird, influences via the
mesenchyme would seem to be logical suspects (compare Cairns and
Saunders, 1954). Sengel (1964) has demonstrated such an influence by an
extract of nervous tissue in the development of the first middorsal pteryla,
whose position, he thought, determined the dorsal pterylar pattern and
therefore the tract margins. This point is discussed further in the light
of his subsequent results (see below).
It has long been believed (e.g., Lillie and Wang, 1943; Wang, 1943;
Cohen, 1961, 1964) that factors resulting in maintenance of tract specificity in the adult bird or mammal reside in the epidermis. The evidence
for this view now seems weak (see Cohen, 1965, for a full discussion),
and it seems that in the adult, as in the embryo, the dermis controls the
mode of differentiation of the epidermis, not only as regards its surface
character, but also as regards appendages. The dermal papillae, however,
are still supposed not to have tract-specific properties or influence (see
below).
The feathers in the tracts of various breeds of fowl, or, indeed, of
various species of bird, often do not seem very similar. Willier and