VII.
LIMB
MORPHOGENESIS
323
and mesoderm. The epidermis moves distally relatively faster than the
mesoderm; a given region of the latter is, therefore, covered by successively different epidermal cells. Another consequence of this relatively
more rapid movement of the epidermis is that the cells 'pile up' at the
marginal ridge and spread along it in a cranio-caudal direction. The
ridge, according to Amprino and Camosso, is thus made up, in part, of
cells which were formerly part of the lateral epidermis. Some of the cells
of the ridge come from local mitotic activity; between the two sources
ridge cells which degenerate (a normal phenomenon in this region) are
replaced.
Conclusions about a proximo-distal sequence of limb part formation
are not restricted to the chick embryo and probably are a valid generalization for all vertebrate limbs. This is indicated by the analogous
findings of Tschumi for the anuran, Xenopus. Tschumi (1955, 1957)
applied carbon marks to limb buds of various stages and obtained
results which are in agreement with those from chick limbs. As might be
expected, the disposition of the presumptive territories of Xenopus limb
buds differs from that seen in the chick, but the basic phenomena are
the same. According to Tschumi the epidermis grows by a fairly uniform
expansion throughout and retains its relative position over the limb;
thus the distal part remains over the distal region and the median part
remains over a median part of the limb even though the underlying
mesoderm may be different because of the relative displacement which
is a consequence of the distal growth of the mesoderm. Denuded limb
mesoderm, grown out of contact with foreign host ectoderm, developed
as did Saunders' ridge-free buds and Balinsky's grafts of similar tissue.
Smith (1958) has shown a similar proximo-distal sequence (and failure
of distal growth) in ectoderm-free limb buds from hamster embryos of
increasing age.
IV. Mapping Presumptive Limb Areas
An important adjunct to the earlier experimental studies of amphibian
limb development was the precise mapping of the site of the presumptive
limb areas. Harrison (1915) located the presumptive fore-limb area in
tail-bud stages of Ambystoma punctatum and Detwiler traced this
presumptive limb tissue back to early neurula and gastrula stages. The
presumptive limb areas of chick embryos have been mapped roughly by
Rudnick (1945), who made intra-coelomic grafts of carefully mapped
regions of one and two day embryos. The grafts which formed limbs or
limb parts were projected onto whole embryos to give some indication
of the regions in which prospective limb areas are located. Chaube
(1959) combined chalk marking of early blastoderms with grafting
experiments to provide a more precise map of prospective limb-forming
LIMB
MORPHOGENESIS
323
and mesoderm. The epidermis moves distally relatively faster than the
mesoderm; a given region of the latter is, therefore, covered by successively different epidermal cells. Another consequence of this relatively
more rapid movement of the epidermis is that the cells 'pile up' at the
marginal ridge and spread along it in a cranio-caudal direction. The
ridge, according to Amprino and Camosso, is thus made up, in part, of
cells which were formerly part of the lateral epidermis. Some of the cells
of the ridge come from local mitotic activity; between the two sources
ridge cells which degenerate (a normal phenomenon in this region) are
replaced.
Conclusions about a proximo-distal sequence of limb part formation
are not restricted to the chick embryo and probably are a valid generalization for all vertebrate limbs. This is indicated by the analogous
findings of Tschumi for the anuran, Xenopus. Tschumi (1955, 1957)
applied carbon marks to limb buds of various stages and obtained
results which are in agreement with those from chick limbs. As might be
expected, the disposition of the presumptive territories of Xenopus limb
buds differs from that seen in the chick, but the basic phenomena are
the same. According to Tschumi the epidermis grows by a fairly uniform
expansion throughout and retains its relative position over the limb;
thus the distal part remains over the distal region and the median part
remains over a median part of the limb even though the underlying
mesoderm may be different because of the relative displacement which
is a consequence of the distal growth of the mesoderm. Denuded limb
mesoderm, grown out of contact with foreign host ectoderm, developed
as did Saunders' ridge-free buds and Balinsky's grafts of similar tissue.
Smith (1958) has shown a similar proximo-distal sequence (and failure
of distal growth) in ectoderm-free limb buds from hamster embryos of
increasing age.
IV. Mapping Presumptive Limb Areas
An important adjunct to the earlier experimental studies of amphibian
limb development was the precise mapping of the site of the presumptive
limb areas. Harrison (1915) located the presumptive fore-limb area in
tail-bud stages of Ambystoma punctatum and Detwiler traced this
presumptive limb tissue back to early neurula and gastrula stages. The
presumptive limb areas of chick embryos have been mapped roughly by
Rudnick (1945), who made intra-coelomic grafts of carefully mapped
regions of one and two day embryos. The grafts which formed limbs or
limb parts were projected onto whole embryos to give some indication
of the regions in which prospective limb areas are located. Chaube
(1959) combined chalk marking of early blastoderms with grafting
experiments to provide a more precise map of prospective limb-forming