FEATHERS AND PATTERNS
11
Rawles (1938a,b, 1940), however, developed a technique for displaying
a surprising similarity at the subtlest level. They implanted tissue containing melanoblasts of various pigmented breeds into the wing buds
of 72-hr White Leghorn embryos (the wing bud of the host embryo is
colonized later, at 80-86 hr, by its own pigment cell stock). Only the
pigment cells from the implants contributed to the host feather. Nevertheless, the color patterns produced in these feathers was that characteristic of the homologous feathers of the donor breed. Even grafts of
American Robin tissue to White Leghorn embryos (Rawles, 1939) produced appropriate pattern in the various wing tracts—anomalies here
may be attributable to the difference in size of the host feathers. It may
be said, therefore, that the intrinsic differences of tract (tract specificities)
seen from breed to breed or even species to species are variations on a
major pattern of tract development and maintenance whose mechanism
may be common to all birds, even in subtle detail.
B. The Pterylae
Hardesty (1933) considered the skin of birds to have a "genetically
determined stencil" to which the various cells and tissues involved in
feather formation responded according to its local nature. Such metaphysical explanations are no longer satisfying. We may now say a little
more about the development of the pterylae, but do not yet have more
than a few vague pointers. Sengel has produced the most elegant work
on this subject, as on many others relating to feathers. He has shown
that the presence of the axial structures of the embryo (spinal cord, myotomes) beneath the dorsal skin seem to be responsible for the appearance
of the first (middorsal) pteryla. Others then appear lateral to it. Here—
as in many morphogenetic problems, especially those concerned with
patterns—the origins of the first programmed heterogeneity is the difficult question; analysis of subsequent elaboration of the system is relatively easy once this has been established (see Gustafson and Wolpert,
1961).
It now seems, from the work of Sengel and Sengel (1965), that the
action of the axial organ implants is not a specific one. The presence of
pieces of Millipore filter, chips of aluminum, and a variety of other
agents may induce accessory pterylae in ventral skin only a little less
reliably than implants of axial organs. The influence, then, is almost certainly a mechanical one ; perhaps the normal pterylae are induced by the
tension in dermis or epidermis of the growing bird as more solid organs
appear in it.
Sengel has shown further that when excised pieces of back skin of
6-7 day embryos are cultured in vitro feather germ rudiments already
11
Rawles (1938a,b, 1940), however, developed a technique for displaying
a surprising similarity at the subtlest level. They implanted tissue containing melanoblasts of various pigmented breeds into the wing buds
of 72-hr White Leghorn embryos (the wing bud of the host embryo is
colonized later, at 80-86 hr, by its own pigment cell stock). Only the
pigment cells from the implants contributed to the host feather. Nevertheless, the color patterns produced in these feathers was that characteristic of the homologous feathers of the donor breed. Even grafts of
American Robin tissue to White Leghorn embryos (Rawles, 1939) produced appropriate pattern in the various wing tracts—anomalies here
may be attributable to the difference in size of the host feathers. It may
be said, therefore, that the intrinsic differences of tract (tract specificities)
seen from breed to breed or even species to species are variations on a
major pattern of tract development and maintenance whose mechanism
may be common to all birds, even in subtle detail.
B. The Pterylae
Hardesty (1933) considered the skin of birds to have a "genetically
determined stencil" to which the various cells and tissues involved in
feather formation responded according to its local nature. Such metaphysical explanations are no longer satisfying. We may now say a little
more about the development of the pterylae, but do not yet have more
than a few vague pointers. Sengel has produced the most elegant work
on this subject, as on many others relating to feathers. He has shown
that the presence of the axial structures of the embryo (spinal cord, myotomes) beneath the dorsal skin seem to be responsible for the appearance
of the first (middorsal) pteryla. Others then appear lateral to it. Here—
as in many morphogenetic problems, especially those concerned with
patterns—the origins of the first programmed heterogeneity is the difficult question; analysis of subsequent elaboration of the system is relatively easy once this has been established (see Gustafson and Wolpert,
1961).
It now seems, from the work of Sengel and Sengel (1965), that the
action of the axial organ implants is not a specific one. The presence of
pieces of Millipore filter, chips of aluminum, and a variety of other
agents may induce accessory pterylae in ventral skin only a little less
reliably than implants of axial organs. The influence, then, is almost certainly a mechanical one ; perhaps the normal pterylae are induced by the
tension in dermis or epidermis of the growing bird as more solid organs
appear in it.
Sengel has shown further that when excised pieces of back skin of
6-7 day embryos are cultured in vitro feather germ rudiments already