322
EDGAR
ZWILLING
removed from the youngest wing buds (equivalent to stage 17 of Hamburger and Hamilton, 1951) elaboration of a wing was suppressed and
only the girdle and the proximal end of the humerus developed. After
the same operation successively older wing buds developed more and
more of the wing, but still failed to form the most distal parts. The wing
elements which did form were those to be expected from the fate maps
which Saunders had been able to make, based on experiments in which
various parts of the wing buds (stages 17-21) were marked with carbon
particles.
In the last mentioned approach small masses of finely powdered
carbon were inserted into the mesoderm of various parts of the limb
buds. Those placed near the base of the bud remained compact and
moved relatively little and were recovered in the girdle elements. Those
placed at the distal tip of the bud usually elongated and were left behind
by the growing tip of the wing. They were recovered somewhere in the
humeral or radio-ulnar elements, depending on the stage of the wing bud
at the time of marking and the exact location of the mark. This data not
only resulted in the presumptive maps but added weighty support to the
ideas that limb parts are formed in a proximo-distal sequence and that
the successive parts result from the development of a rather small zone of
apical growth. In addition, Saunders emphasized the importance of the
apical ridge for the continued elaboration of the limb and insisted that
it is '. . .an essential part of an integrated system . . .' which produces a
normal limb. This has been discussed more fully above. (See, however,
Milaire's criticism of carbon marking as a basis for following tissue
movements).
Carbon marking of the chick's leg bud (Hampé, 1956b) revealed that
essentially the same sequence of proximo-distal limb elaboration occurs
in this limb. There are minor differences, such as the axis of inclination
of the presumptive bone elements (i.e. the presumptive radio-ulnar
territories are oblique from post- to pre-axially while the presumptive
tibio-fibular territories are oblique from pre- to post-axially) but the
maps show good basic agreement. Amprino and Camosso (1956, 1958c)
performed an extensive series of experiments in which both the ectoderm
and the mesoderm of wing buds were marked with finely powdered
coloured chalk. The map of presumptive parts based on their work
indicates that the presumptive carpal and hand territories are laid down
somewhat later than indicated by Saunders. These territories are present
in stage 20-21 wing buds according to Saunders but Amprino and
Camosso do not find them complete until stage 24-27 (the distal part of
a stage 22 wing bud represents, according to Amprino and Camosso, the
presumptive carpal territory). The latter authors have observed a rather
important difference in the relative rates of movement of the epidermis
EDGAR
ZWILLING
removed from the youngest wing buds (equivalent to stage 17 of Hamburger and Hamilton, 1951) elaboration of a wing was suppressed and
only the girdle and the proximal end of the humerus developed. After
the same operation successively older wing buds developed more and
more of the wing, but still failed to form the most distal parts. The wing
elements which did form were those to be expected from the fate maps
which Saunders had been able to make, based on experiments in which
various parts of the wing buds (stages 17-21) were marked with carbon
particles.
In the last mentioned approach small masses of finely powdered
carbon were inserted into the mesoderm of various parts of the limb
buds. Those placed near the base of the bud remained compact and
moved relatively little and were recovered in the girdle elements. Those
placed at the distal tip of the bud usually elongated and were left behind
by the growing tip of the wing. They were recovered somewhere in the
humeral or radio-ulnar elements, depending on the stage of the wing bud
at the time of marking and the exact location of the mark. This data not
only resulted in the presumptive maps but added weighty support to the
ideas that limb parts are formed in a proximo-distal sequence and that
the successive parts result from the development of a rather small zone of
apical growth. In addition, Saunders emphasized the importance of the
apical ridge for the continued elaboration of the limb and insisted that
it is '. . .an essential part of an integrated system . . .' which produces a
normal limb. This has been discussed more fully above. (See, however,
Milaire's criticism of carbon marking as a basis for following tissue
movements).
Carbon marking of the chick's leg bud (Hampé, 1956b) revealed that
essentially the same sequence of proximo-distal limb elaboration occurs
in this limb. There are minor differences, such as the axis of inclination
of the presumptive bone elements (i.e. the presumptive radio-ulnar
territories are oblique from post- to pre-axially while the presumptive
tibio-fibular territories are oblique from pre- to post-axially) but the
maps show good basic agreement. Amprino and Camosso (1956, 1958c)
performed an extensive series of experiments in which both the ectoderm
and the mesoderm of wing buds were marked with finely powdered
coloured chalk. The map of presumptive parts based on their work
indicates that the presumptive carpal and hand territories are laid down
somewhat later than indicated by Saunders. These territories are present
in stage 20-21 wing buds according to Saunders but Amprino and
Camosso do not find them complete until stage 24-27 (the distal part of
a stage 22 wing bud represents, according to Amprino and Camosso, the
presumptive carpal territory). The latter authors have observed a rather
important difference in the relative rates of movement of the epidermis