VII.
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MORPHOGENESIS
313
between the toes of the feet which developed from these grafts (Zwilling,
1956d, 1959). Hampé (1957a) did not obtain the same results in somewhat similar experiments; but his procedure involved exchanges between
distal tips of limb buds and not the entire ectodermal covering. Onlytwo grafts succeeded in forming digits and these had no webbing. In my
experiments webbing was found between at least two digits in nine of
eleven duck ectoderm and chick mesoderm chimaeras.
We have now a rather important list of circumstances in which the
ectodermal ridge makes a positive contribution to limb development.
This gives us uncontroverted evidence of what limb ectoderm (ridge) is
capable of doing. The important question is—does the ectodermal ridge
exert these capabilities during normal morphogenesis or does it play a
passive role and submerge its morphogenetic talents and keep them in
reserve for the experimenter only? We can never be certain about what
takes place in normal development. Each time that an experiment is
performed there is some type of interference with the normal situation.
But without experiments we have no rational basis for deducing what
transpires. Any deductions, however, should be based on a carefully
weighed consideration of all of the evidence.
How, then, shall we evaluate the contradictory claims about the
importance of the ectoderm in limb development? There are known
situations in which a structure, which normally forms after an inductive
stimulus, may differentiate in the absence of the initiating tissue. A lens
may form from presumptive lens tissue which has had no contact with
an eye cup under certain circumstances (Spemann, 1938; Ten Cate,
1953). This phenomenon has been named 'double assurance'. Ordinarily,
metanephric kidney tubules form when the ureteric duct tissue makes
contact with the metanephrogenous tissue. Tubules may develop from
mouse embryo metanephrogenous tissue in the absence of contact with
the ureteric duct, in such sites as the anterior eye chamber, brain or
subcutaneous tissue of an adult mouse or coelom of a four day chick
embryo (Grobstein and Parker, 1958). These exceptional cases do not
vitiate the importance, in normal development, of the usual inductive
system any more than does the differentiation, under some circumstances, of neural tissue from isolated presumptive neural ectoderm
detract from the importance of the roof of the archenteron for the
induction of brain.
Before we succumb to the temptation to dismiss the limb situation as
another possible case of double assurance, though, we should examine
the negative evidence a bit more carefully. Amprino and Camosso (1955a,
b) have confirmed Saunders' observation that distal wing structures do
not develop after removal of the ectodermal ridge. They base a good
deal of their argument against the importance of the ridge on the fact
LIMB
MORPHOGENESIS
313
between the toes of the feet which developed from these grafts (Zwilling,
1956d, 1959). Hampé (1957a) did not obtain the same results in somewhat similar experiments; but his procedure involved exchanges between
distal tips of limb buds and not the entire ectodermal covering. Onlytwo grafts succeeded in forming digits and these had no webbing. In my
experiments webbing was found between at least two digits in nine of
eleven duck ectoderm and chick mesoderm chimaeras.
We have now a rather important list of circumstances in which the
ectodermal ridge makes a positive contribution to limb development.
This gives us uncontroverted evidence of what limb ectoderm (ridge) is
capable of doing. The important question is—does the ectodermal ridge
exert these capabilities during normal morphogenesis or does it play a
passive role and submerge its morphogenetic talents and keep them in
reserve for the experimenter only? We can never be certain about what
takes place in normal development. Each time that an experiment is
performed there is some type of interference with the normal situation.
But without experiments we have no rational basis for deducing what
transpires. Any deductions, however, should be based on a carefully
weighed consideration of all of the evidence.
How, then, shall we evaluate the contradictory claims about the
importance of the ectoderm in limb development? There are known
situations in which a structure, which normally forms after an inductive
stimulus, may differentiate in the absence of the initiating tissue. A lens
may form from presumptive lens tissue which has had no contact with
an eye cup under certain circumstances (Spemann, 1938; Ten Cate,
1953). This phenomenon has been named 'double assurance'. Ordinarily,
metanephric kidney tubules form when the ureteric duct tissue makes
contact with the metanephrogenous tissue. Tubules may develop from
mouse embryo metanephrogenous tissue in the absence of contact with
the ureteric duct, in such sites as the anterior eye chamber, brain or
subcutaneous tissue of an adult mouse or coelom of a four day chick
embryo (Grobstein and Parker, 1958). These exceptional cases do not
vitiate the importance, in normal development, of the usual inductive
system any more than does the differentiation, under some circumstances, of neural tissue from isolated presumptive neural ectoderm
detract from the importance of the roof of the archenteron for the
induction of brain.
Before we succumb to the temptation to dismiss the limb situation as
another possible case of double assurance, though, we should examine
the negative evidence a bit more carefully. Amprino and Camosso (1955a,
b) have confirmed Saunders' observation that distal wing structures do
not develop after removal of the ectodermal ridge. They base a good
deal of their argument against the importance of the ridge on the fact