AUTHOR’S ABSTRACT OF THIS PAPER ISSUED 
BY THE BIBLIOGRAPHIC SERVICE, MARCH 30 
EXPERIMENTS ON THE DEVELOPMENT OF THE 
FORE LIMB OF AMBLYSTOMA, A SELF-DIFFER- 
ENTIATING EQUIPOTENTIAL SYSTEM 
ROSS G. HARRISON 
Osborn Zoblogical Laboratory, Yale University 
FORTY-FIVE FIGURES 
Since Balfour’s (78) discovery of muscle buds in the develop- 
ing fins of elasmobranchs the composite nature of the vertebrate 
limb has been emphasized. In the lower forms at least five 
different embryonal tissues are involved in its make-up, and 
these recur in part metamerically. Primarily there is a local 
thickening of the somatopleure which gives rise to the main 
bulk of the limb—the skeleton and other connective-tissue ele- 
ments. This is covered over by ectoderm which in that region 
shows distinct modification. Into the mesenchymal mass de- 
rived from the somatopleure the muscle buds, blood vessels and 
nerves grow sooner or later from the trunk of the embryo. 
Some years ago it was pointed out (Harrison, 95) that the 
various fins of the salmon form a distinct series as regards orig- 
inal visible complexity, beginning with the median fins where 
metameric muscle buds are clearly formed, as in the elasmo- 
branchs, and ending with the pectoral fin, which receives no 
muscle buds. Here the proliferation of the somatopleure forms a 
blastema which gives rise to the muscles as well as to the skeletal 
elements.!_. As compared with the primitive elasmobranch fin, the 
pectoral fin is thus a more compact and unified organ at the time 
of its origin, and this is true in even higher degree for both ex- 
tremities in the Amphibia. Probably also -the limbs of the 
1 Derjugin (’08) has described what purport to be muscle buds in the pectoral 
fins of Exocoetus volitans. In view of the differences in the fins of a single species, 
Salmo salar, it would not be strange, were specific differences of this kind to be 
found in the pectoral fin of various bony fishes. 
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THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 25, NO. 2 
