VII.
LIMB MORPHOGENESIS
325
much of the system is left intact or can be re-established. When a limb
bud is divided by an incision which cuts it into anterior and posterior
halves each of the fragments may behave like a mosaic. In this situation
the missing regions of the ridge do not regenerate and the partial
pattern is maintained (Warren, 1934). Similar results may be obtained
when the line of incision is parallel to the long axis of the bud and
removes the entire distal growing tip (Selby and Murray, 1928; Saunders,
1948). Each part develops according to its prospective fate. Some of the
first experiments to indicate regulative (non-mosaic) properties of the
chick limb system were performed on presumptive limb areas. Coelomic
grafts of such early tissues may produce duplicated limb parts (Rudnick,
1945). Transverse incisions across the hind-limb field have also resulted
in the development of accessory limb structures (Wolff and Kahn, 1947)
and some of Kieny's (1959) flank grafts of pre-limb bud mesoderm
developed supernumerary digits. However, even limb buds may evince
considerable regulability. This is particularly evident when the central
portions of the limb bud are altered and the distal ridge-mesoderm
complex is left intact. Wolff and Hampé (1954) and Hampé (1956c,
1957b, 1958) have been concerned with such situations. When a leg bud
was divided into three pieces by cuts parallel to the antero-posterior
axis, the median piece eliminated and the distal section fused to the
proximal part, a nearly normal limb developed, complete with a
femoro-tibial joint but without a fibula. The median piece, grown on a
C-A membrane, formed a second femora-tibial joint. When an entire leg
bud of stage 18 or 19 was fused to the distal end of a stage 21-22 bud
from which the distal tip had been removed an almost normal leg
developed. The only striking variation from the normal was in the
fibula, which was as long and well developed as a tibia. Both the base of
the host limb and the grafted leg buds have presumptive girdle, femur,
etc. present, but these regulate to form an harmonious whole. The effect
of deficiency or excess of tissue on absence or over-development of the
fibula has been considered (Hampé, 1958) to be a reflection of its evolutionary instability in various birds.
Amprino and Camosso (1955a), have performed an extensive study of
regulation after removal of various regions of wing buds in different
stages of development. They confirm some of the statements made
above. Their deletion of anterior and proximal limb bud halves (stages
19-20) gave essentially the results obtained by Warren. Deletion
of as much as 20% of the medial portion of limb buds as old as
stage 24 was followed by virtually complete regulation. This last finding
was corroborated and extended by Hansborough (1956) in similar
experiments on polydactylous wing buds. 30% or more of the medial
tissue was removed and an essentially normal wing developed. As long
LIMB MORPHOGENESIS
325
much of the system is left intact or can be re-established. When a limb
bud is divided by an incision which cuts it into anterior and posterior
halves each of the fragments may behave like a mosaic. In this situation
the missing regions of the ridge do not regenerate and the partial
pattern is maintained (Warren, 1934). Similar results may be obtained
when the line of incision is parallel to the long axis of the bud and
removes the entire distal growing tip (Selby and Murray, 1928; Saunders,
1948). Each part develops according to its prospective fate. Some of the
first experiments to indicate regulative (non-mosaic) properties of the
chick limb system were performed on presumptive limb areas. Coelomic
grafts of such early tissues may produce duplicated limb parts (Rudnick,
1945). Transverse incisions across the hind-limb field have also resulted
in the development of accessory limb structures (Wolff and Kahn, 1947)
and some of Kieny's (1959) flank grafts of pre-limb bud mesoderm
developed supernumerary digits. However, even limb buds may evince
considerable regulability. This is particularly evident when the central
portions of the limb bud are altered and the distal ridge-mesoderm
complex is left intact. Wolff and Hampé (1954) and Hampé (1956c,
1957b, 1958) have been concerned with such situations. When a leg bud
was divided into three pieces by cuts parallel to the antero-posterior
axis, the median piece eliminated and the distal section fused to the
proximal part, a nearly normal limb developed, complete with a
femoro-tibial joint but without a fibula. The median piece, grown on a
C-A membrane, formed a second femora-tibial joint. When an entire leg
bud of stage 18 or 19 was fused to the distal end of a stage 21-22 bud
from which the distal tip had been removed an almost normal leg
developed. The only striking variation from the normal was in the
fibula, which was as long and well developed as a tibia. Both the base of
the host limb and the grafted leg buds have presumptive girdle, femur,
etc. present, but these regulate to form an harmonious whole. The effect
of deficiency or excess of tissue on absence or over-development of the
fibula has been considered (Hampé, 1958) to be a reflection of its evolutionary instability in various birds.
Amprino and Camosso (1955a), have performed an extensive study of
regulation after removal of various regions of wing buds in different
stages of development. They confirm some of the statements made
above. Their deletion of anterior and proximal limb bud halves (stages
19-20) gave essentially the results obtained by Warren. Deletion
of as much as 20% of the medial portion of limb buds as old as
stage 24 was followed by virtually complete regulation. This last finding
was corroborated and extended by Hansborough (1956) in similar
experiments on polydactylous wing buds. 30% or more of the medial
tissue was removed and an essentially normal wing developed. As long