6
JACK COHEN
better example is the distinction between wing or tail primaries and their
coverts. Figure 1 shows the location of such tracts of feathers on a fowl.
All the feathers in one tract are not alike, however. In a median tract,
feathers to left and right of the midline will frequently show mirrorimage asymmetry both in their structure and in their arrangement. Lateral tracts (e.g., wing coverts) may show the same phenomenon, but less
exactly. (Some lateral tracts, e.g., the wing primaries, show almost
perfect mirror imagery in their asymmetry; this is probably important
for flight.)
Feathers from different tracts differ not only in their morphology, but
also usually in their pigmentation (Fig. 2). Furthermore, the time in the
life history at which the follicles of the various tracts become competent
to react to hormones, by producing feathers of different color, varies for
the different tracts (Trinkaus, 1953) and may be correlated with the
order of their embryonic development (Holmes, 1935; Cohen, 1957).
When the bird molts, too, there is a characteristic order of feather loss
in the various tracts; this order is more or less the reverse of the order
of development of the tracts (Holmes, 1935), as in the rat (Ebling and
Johnson, 1664). The scales on the legs of some birds (e.g., chickens) and
the bare areas under the wing should also be regarded as tracts.
B. The Pterylae
Within each tract the feather follicles are usually arranged in rows or
pterylae. During embryonic development such rows appear first quite
widely spaced (the primary pterylae) ; later secondary pterylae appear
between them, and tertiary and quaternary pterylae may appear later
still. It is probable, however, that few if any new follicles are added
after hatching in most species (Lillie, 1942). Most birds have sparse
feathers high on the flanks under the wings, and these follicles do not
seem to develop in pterylae, but in more random fashion (Holmes, 1935).
The same may be true of the cloacal area and the underside of the
humeral part of the wing. The scales on the legs do have a regular
pattern, but this probably results at least in part from their overlapping
(imbrication)—the pattern of their development may not be so regular.
Rawles (1963) presents experiments of extraordinary elegance as well as
a review of scale and feather development.
Pterylae usually develop in an anteroposterior direction, but their
position may later be modified as the bird changes in shape while
maturing. This is especially so on the flanks and the limbs, as might be
expected.
JACK COHEN
better example is the distinction between wing or tail primaries and their
coverts. Figure 1 shows the location of such tracts of feathers on a fowl.
All the feathers in one tract are not alike, however. In a median tract,
feathers to left and right of the midline will frequently show mirrorimage asymmetry both in their structure and in their arrangement. Lateral tracts (e.g., wing coverts) may show the same phenomenon, but less
exactly. (Some lateral tracts, e.g., the wing primaries, show almost
perfect mirror imagery in their asymmetry; this is probably important
for flight.)
Feathers from different tracts differ not only in their morphology, but
also usually in their pigmentation (Fig. 2). Furthermore, the time in the
life history at which the follicles of the various tracts become competent
to react to hormones, by producing feathers of different color, varies for
the different tracts (Trinkaus, 1953) and may be correlated with the
order of their embryonic development (Holmes, 1935; Cohen, 1957).
When the bird molts, too, there is a characteristic order of feather loss
in the various tracts; this order is more or less the reverse of the order
of development of the tracts (Holmes, 1935), as in the rat (Ebling and
Johnson, 1664). The scales on the legs of some birds (e.g., chickens) and
the bare areas under the wing should also be regarded as tracts.
B. The Pterylae
Within each tract the feather follicles are usually arranged in rows or
pterylae. During embryonic development such rows appear first quite
widely spaced (the primary pterylae) ; later secondary pterylae appear
between them, and tertiary and quaternary pterylae may appear later
still. It is probable, however, that few if any new follicles are added
after hatching in most species (Lillie, 1942). Most birds have sparse
feathers high on the flanks under the wings, and these follicles do not
seem to develop in pterylae, but in more random fashion (Holmes, 1935).
The same may be true of the cloacal area and the underside of the
humeral part of the wing. The scales on the legs do have a regular
pattern, but this probably results at least in part from their overlapping
(imbrication)—the pattern of their development may not be so regular.
Rawles (1963) presents experiments of extraordinary elegance as well as
a review of scale and feather development.
Pterylae usually develop in an anteroposterior direction, but their
position may later be modified as the bird changes in shape while
maturing. This is especially so on the flanks and the limbs, as might be
expected.