III.
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129
the dermis. The other role of the skin, namely, that of promoting
regeneration, is probably due to influences exerted by the epidermis.
VI. Regeneration from Quantitatively Altered Appendages
In addition to the effects of specific tissues on the differentiation of
the blastema, it is important to understand something of the factors
responsible for the completeness of the regenerate. To this end, many
of the experimental approaches applied to individual tissues can be
adapted to entire extremities. Thus, with special reference to the degree
of wholeness of limb regenerates, it is of particular interest to investigate
the regeneration of double limbs, half limbs, extremities compounded
with parts of other appendages, legs with their component tissues
minced and scrambled, and structures which have been partially
irradiated. These experiments and their variations, have yielded
valuable information concerning the role of the stump in determining
the total organization of the regenerate.
A. Regeneration of Double Limbs
If two limbs are grafted together in such a way that upon amputation
the cut surface exposes twice the normal amount of limb tissue, will this
compound limb give rise to a single or a double regenerate? Experiments
of this kind have proved that under these conditions the resulting
regenerate is originally double, but that some fusion between the two
components often occurs. Weiss (1924b) first did such an experiment by
flexing the lower arm upon the upper arm and grafting the two together,
whereupon amputation was performed just proximal to the elbow.
Although the lower arm component of such a double limb was without
innervation and was oriented backwards, the subsequent regenerates
contained paired hands with digits arranged linearly. Where the two
hands fused with each other the number of fingers was less than twice
the normal number, reflecting the fact that some had either fused
together into single digits or were entirely missing. Comparable results
have since been obtained by Monroy (1943) and Goss (1956d) after
grafting arms together in symmetrical orientations. Left and right arms
grafted together side-by-side or in ventral-to-ventral arrangements gave
rise to partially duplicated regenerates which had undergone varying
degrees of fusion during their development. However, it is evident that
in order for two structures to fuse together into a single entity, certain
parts of each must be obliterated in the process. Thus, single hand
regenerates made up of two fused hands in a palm-to-palm orientation,
are neither left nor right hands, but a mixture of both. Such a compound
hand has two dorsal surfaces but no palm, and in this case, the ventral
REGENERATION OF VERTEBRATE APPENDAGES
129
the dermis. The other role of the skin, namely, that of promoting
regeneration, is probably due to influences exerted by the epidermis.
VI. Regeneration from Quantitatively Altered Appendages
In addition to the effects of specific tissues on the differentiation of
the blastema, it is important to understand something of the factors
responsible for the completeness of the regenerate. To this end, many
of the experimental approaches applied to individual tissues can be
adapted to entire extremities. Thus, with special reference to the degree
of wholeness of limb regenerates, it is of particular interest to investigate
the regeneration of double limbs, half limbs, extremities compounded
with parts of other appendages, legs with their component tissues
minced and scrambled, and structures which have been partially
irradiated. These experiments and their variations, have yielded
valuable information concerning the role of the stump in determining
the total organization of the regenerate.
A. Regeneration of Double Limbs
If two limbs are grafted together in such a way that upon amputation
the cut surface exposes twice the normal amount of limb tissue, will this
compound limb give rise to a single or a double regenerate? Experiments
of this kind have proved that under these conditions the resulting
regenerate is originally double, but that some fusion between the two
components often occurs. Weiss (1924b) first did such an experiment by
flexing the lower arm upon the upper arm and grafting the two together,
whereupon amputation was performed just proximal to the elbow.
Although the lower arm component of such a double limb was without
innervation and was oriented backwards, the subsequent regenerates
contained paired hands with digits arranged linearly. Where the two
hands fused with each other the number of fingers was less than twice
the normal number, reflecting the fact that some had either fused
together into single digits or were entirely missing. Comparable results
have since been obtained by Monroy (1943) and Goss (1956d) after
grafting arms together in symmetrical orientations. Left and right arms
grafted together side-by-side or in ventral-to-ventral arrangements gave
rise to partially duplicated regenerates which had undergone varying
degrees of fusion during their development. However, it is evident that
in order for two structures to fuse together into a single entity, certain
parts of each must be obliterated in the process. Thus, single hand
regenerates made up of two fused hands in a palm-to-palm orientation,
are neither left nor right hands, but a mixture of both. Such a compound
hand has two dorsal surfaces but no palm, and in this case, the ventral