314
EDGAR
ZWILLING
that more limb developed when ridge and underlying mesoderm were
excised than when ridge alone was removed. The most critical pertinent
operations were those (Amprino and Camosso, 1955a, cf. Fig. Id) in
which the apical ridge plus subjacent mesoderm were removed from the
distal tips of stage 20-21 limb buds. Three wing buds and two leg buds
were treated this way. Digits and other distal parts were missing when
these buds completed development but the long bones, including the
metacarpals (reduced in size), were formed more completely than when
all of the ridge alone or with a very small amount of mesoderm was
removed (20 cases). It is of some interest that these results are almost
the same as those which they obtained when they removed a large
median region (ectoderm and mesoderm) of limb buds of the same
stage but left a small amount of ectodermal ridge on either side of
the excision (cf. their Fig. lc). It is quite understandable that some
regulation may occur as long as some distal outgrowth (at either end
of the bud) may continue (see also Rudnick, 1946) and that tissue
may fill in the gap between the two regions of outgrowth in their
lc cases. It seems quite likely that their Id cases resulted from outgrowths of the same sort and were due to residual ridge. As Saunders
et al. (1957) have pointed out, a relatively small portion of the post-axial
ridge is normally involved in the development of a relatively large
proportion of the distal structures. With the possibility of some regulation more of the levels proximal to the digits (i.e. metacarpals) may form
from this region than is expected. However, it is important to note that
digits did not develop even in these critical cases of Amprino and
Camosso. Amprino and Camosso (1955b) have reported 120 additional
operations, presumably the same as type Id (above), performed on limb
buds of stage 18-20. The authors report that 50 of these produced a
'regulation totale ou subtotale'. Only 30 of the limb buds did not regulate at all. Without information about precisely how much of the postaxial ridge was left in situ and how much of regulation was 'subtotale'
it is difficult to evaluate these observations.
The non-conforming results obtained by Bell and co-workers with
focused ultra-sound may be, in the final analysis, quite important to our
understanding of the mechanism of limb development. There are at
least three possible ways to account for his observations. 1. Removal of
the ectoderm by ultrasonation may be less damaging than either surgical
or chemical procedures and thus allow for more normal morphogenesis.
2. The ultrasonation may impart some property to the mesoblast which
endows it with an unusual independence from the ectoderm. 3. This
treatment may leave an essential part or product of the ectoderm on
the mesoblast. In an attempt to resolve some of these possibilities Bell,
Saunders and Zwilling have undertaken some joint experiments. Thus
EDGAR
ZWILLING
that more limb developed when ridge and underlying mesoderm were
excised than when ridge alone was removed. The most critical pertinent
operations were those (Amprino and Camosso, 1955a, cf. Fig. Id) in
which the apical ridge plus subjacent mesoderm were removed from the
distal tips of stage 20-21 limb buds. Three wing buds and two leg buds
were treated this way. Digits and other distal parts were missing when
these buds completed development but the long bones, including the
metacarpals (reduced in size), were formed more completely than when
all of the ridge alone or with a very small amount of mesoderm was
removed (20 cases). It is of some interest that these results are almost
the same as those which they obtained when they removed a large
median region (ectoderm and mesoderm) of limb buds of the same
stage but left a small amount of ectodermal ridge on either side of
the excision (cf. their Fig. lc). It is quite understandable that some
regulation may occur as long as some distal outgrowth (at either end
of the bud) may continue (see also Rudnick, 1946) and that tissue
may fill in the gap between the two regions of outgrowth in their
lc cases. It seems quite likely that their Id cases resulted from outgrowths of the same sort and were due to residual ridge. As Saunders
et al. (1957) have pointed out, a relatively small portion of the post-axial
ridge is normally involved in the development of a relatively large
proportion of the distal structures. With the possibility of some regulation more of the levels proximal to the digits (i.e. metacarpals) may form
from this region than is expected. However, it is important to note that
digits did not develop even in these critical cases of Amprino and
Camosso. Amprino and Camosso (1955b) have reported 120 additional
operations, presumably the same as type Id (above), performed on limb
buds of stage 18-20. The authors report that 50 of these produced a
'regulation totale ou subtotale'. Only 30 of the limb buds did not regulate at all. Without information about precisely how much of the postaxial ridge was left in situ and how much of regulation was 'subtotale'
it is difficult to evaluate these observations.
The non-conforming results obtained by Bell and co-workers with
focused ultra-sound may be, in the final analysis, quite important to our
understanding of the mechanism of limb development. There are at
least three possible ways to account for his observations. 1. Removal of
the ectoderm by ultrasonation may be less damaging than either surgical
or chemical procedures and thus allow for more normal morphogenesis.
2. The ultrasonation may impart some property to the mesoblast which
endows it with an unusual independence from the ectoderm. 3. This
treatment may leave an essential part or product of the ectoderm on
the mesoblast. In an attempt to resolve some of these possibilities Bell,
Saunders and Zwilling have undertaken some joint experiments. Thus