OBSERVATIONS ON THE ORIGIN OF THE PAIRED LIMBS 
OF VERTEBRATES. 
BY 
RAYMOND C. CSBURN. 
From the Zoological Laboratory of Columbia University. 
WITH 5 PLATES. 
Perhaps in no field of Vertebrate zodlogy have two diametrically oppo- 
site views been more steadily and skillfully maintained than in that which 
forms the subject of the present paper. Disregarding the earlier specu- 
lations in regard to the origin of the paired hmbs, we may date the real 
work in this field from the appearance of Gegenhbaur’s first paper on the 
subject in 1865. This prolific student adopted the earlier suggestion of 
Owen that the limb girdles correspond to misplaced gill-arches and pro- 
ceeded to defend this view with such vigor and resource that it was for 
many years the accepted hypothesis. At the same time the unpaired 
fin skeleton was held to be a derivative of the axial skeleton. These views 
were steadily maintained by Gegenbaur through thirty years of investi- 
gation, indeed, up to the appearance of his last paper on the subject in 
1895. His students have also contributed much to the support of the 
hypothesis. * 
Opposed to the gill-arch theory (Gegenbaur theory, archipterygium 
theory) is that known as the fin-fold, or lateral-fold theory, which postu- 
lates a similar origin for the paired and unpaired fins as local outgrowths 
from the body-wall, independent of gills, axial skeleton, or any other 
structures. This view was first developed in the researches of Thacher, 
77, Balfour, 78, and Mivart, 79, all working independently, and has be- 
come pretty generally accepted by zodlogists, other than those of the 
Gegenbaur school. Very much has been added to the support of the 
theory since it was first propounded, but, in spite of the exploitation of 
anatomical, paleontological, and embryological fields there have always 
remained numerous ambiguous points, the interpretation of which de- 
pended on the attitude with which they were approached. That the 
problem was not fully settled by the older investigators witness at least 
AMERICAN JOURNAL OF ANATOMY.—VOL. VII. 
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