VII.
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303
progresses distally so that shortly after the paddle appears the cartilaginous precursors of the digits become evident. Subsequent development
of a limb involves the further elaboration of the bone elements, their
growth in length by typical epiphyseal addition, the elongation of the
digits which is accompanied by loss of the interdigital tissue, and the
eventual elaboration of a fully formed appendage.
B. Experimental Background
Advances in our knowledge of limb morphogenesis have taken place
against a background which is, by now, classical and has been reviewed
in many places (Harrison, 1918; Swett, 1927; Nicholas, 1955). Most of
the early experimental analyses of limb development were concerned
with amphibia and fell unequally into two categories. A majority of the
experimental observations were made on presumptive limb-bud tissues
(the so-called limb discs) while a relatively small number of experiments
were carried out with limb buds. With a few exceptions the data obtained with presumptive limb tissues are quite consistent, and may be
summarized as follows:
(a) Initially a much greater area is capable of forming limb structures
than actually does so. In other words, there is a broad limb field.
(6) Presumptive limb tissues stabilize in the direction of limb formation (i.e. are 'determined') relatively early. Because of differences
in time sequence the exact time of stabilization depends on which limb
(fore or hind) is being considered and whether a urodele or an anuran is
studied. The criterion upon which this evidence was largely based was
the ability of the limb system to self-differentiate.
(c) The properties of the limb appear to stabilize in a series of steps:
limb type (fore or hind) stabilizes very early. The antero-posterior axis
becomes stable while the dorso-ventral axis is still subject to modification. Subsequently the dorso-ventral axis becomes stable.
(d) Presumptive limb regions may be traced back to given sectors
(quadrants) of the limb disc, but results of a variety of experiments
emphasize the equipotentiality of the early limb system. An excised
portion of the presumptive limb tissue may form a complete limb when
grafted to a foreign site, while another complete limb may form from the
tissue remaining at the original limb site. Under some circumstances
(e.g. rotation of the limb disc) there is a tendency for the limbs to be
duplicated, i.e. two or more limbs may form.
(e) Most of the important limb properties of the early presumptive
tissues reside in the mesoderm. Isolated mesoderm from these stages
is not capable of forming limb structures; a covering of ectoderm is
required for this. However, ectoderms from a variety of sources are
LIMB MORPHOGENESIS
303
progresses distally so that shortly after the paddle appears the cartilaginous precursors of the digits become evident. Subsequent development
of a limb involves the further elaboration of the bone elements, their
growth in length by typical epiphyseal addition, the elongation of the
digits which is accompanied by loss of the interdigital tissue, and the
eventual elaboration of a fully formed appendage.
B. Experimental Background
Advances in our knowledge of limb morphogenesis have taken place
against a background which is, by now, classical and has been reviewed
in many places (Harrison, 1918; Swett, 1927; Nicholas, 1955). Most of
the early experimental analyses of limb development were concerned
with amphibia and fell unequally into two categories. A majority of the
experimental observations were made on presumptive limb-bud tissues
(the so-called limb discs) while a relatively small number of experiments
were carried out with limb buds. With a few exceptions the data obtained with presumptive limb tissues are quite consistent, and may be
summarized as follows:
(a) Initially a much greater area is capable of forming limb structures
than actually does so. In other words, there is a broad limb field.
(6) Presumptive limb tissues stabilize in the direction of limb formation (i.e. are 'determined') relatively early. Because of differences
in time sequence the exact time of stabilization depends on which limb
(fore or hind) is being considered and whether a urodele or an anuran is
studied. The criterion upon which this evidence was largely based was
the ability of the limb system to self-differentiate.
(c) The properties of the limb appear to stabilize in a series of steps:
limb type (fore or hind) stabilizes very early. The antero-posterior axis
becomes stable while the dorso-ventral axis is still subject to modification. Subsequently the dorso-ventral axis becomes stable.
(d) Presumptive limb regions may be traced back to given sectors
(quadrants) of the limb disc, but results of a variety of experiments
emphasize the equipotentiality of the early limb system. An excised
portion of the presumptive limb tissue may form a complete limb when
grafted to a foreign site, while another complete limb may form from the
tissue remaining at the original limb site. Under some circumstances
(e.g. rotation of the limb disc) there is a tendency for the limbs to be
duplicated, i.e. two or more limbs may form.
(e) Most of the important limb properties of the early presumptive
tissues reside in the mesoderm. Isolated mesoderm from these stages
is not capable of forming limb structures; a covering of ectoderm is
required for this. However, ectoderms from a variety of sources are