100 BULLETIN OF THE 
terior pair of fins develop much more rapidly than those of the posterior 
pair; (5) the connecting ridge of ectoderm between the rudiments of 
the paired fins disappears; (6) these ectodermal rudiments develop 
into elongated projecting folds; (7) the mesoderm at the base of these 
folds becomes closely packed, and forms a slight projection ; (8) the 
cells of both layers of the ectoderm, judging from Balfour’s drawings 
(78, Plate XI. Fig. 9, and Plate XII. Fig. 1), take on a columnar form 
in the regions of the fin-folds. 
It is evident that Balfour regarded the primary modification of the 
ectoderm into a thickening or ridge as the earliest step in the develop- 
ment of the paired fin in Elasmobranchs, and the modification of the 
mesoderm as a secondary condition, and coincident with the formation of 
a distinct fold by the ectoderm. Balfour (’81, Vol. IL. p. 500) also held 
the same view concerning the similar modification of the ectoderm in 
other groups of fishes. 
During the course of the further development, the mesoderm passes 
into the fold of ectoderm, and assumes the appearance of a closely packed 
mass of indifferent cells, which, together with its ectodermal covering, 
now presenting a somewhat rounded contour, constitutes what is known 
as the primitive “limb-bud.” Subsequently the muscle-plates grow in 
a ventral direction to the level of the limb-bud, and several of them turn 
slightly outward at their distal ends, and give off small masses of cells, 
the myotome-buds, which pass into the blastema of the limb-bud, 
where they soon lose their former distinctness, but presumably consti- 
tute the source of the musculature of the limb. The muscle-plates, 
after having given off the myotome-buds, lose all trace of this modifi- 
cation, and continue their growth in the ventral direction. 
Balfour did not determine the real character of the myotome-buds, or 
the number for each myotome ; nor did he trace them to their subsequent 
fate. This, however, was accomplished by Dohrn a few years later. 
Kolliker (79, p. 805) held that the muscle-plates as such in no case 
grew into the limb-buds, and that the view of an independent formation 
of the limb-muscles is provisionally quite as correct as the other view, 
although Remak (’50-’55) had shown that in the Rabbit the muscle- 
plates extended at least a short distance into the limb-bud. 
Dohrn (’84, p. 161) did not investigate the primary source of the 
mesoderm which passes into the ectodermal fin-fold, but, accepting Bal- 
four’s account of the origin of the limb-bud, began his studies with the 
secondary contributions of the mesodermal elements from the muscle- 
plates. Dohrn pointed out that in the region of the pectoral fin, in the 
