190 Origin of the Paired Limbs of Vertebrates 
must be urged that at a still earher stage the mesenchyme thickening from 
which the fin skeleton arises has its origin next to the ectoderm within 
the developing fin-fold. In Cestracion, according to the writer’s observa- 
tions, the first part to appear as procartilage is distal to the girdle and 
includes a portion of the primary basal and the proximal ends of the fin- 
rays in the forward half of the fin. From this point the process of differ- 
entiation spreads in all directions and soon the ventral half of the girdle 
makes its appearance. This is after the fashion which has been described 
by the older embryologists. Fiirbringer, 02, and Braus, oq4a, have used 
Molliers’ figures (1893, Pl. III, Fig. 13, and Pl.. IV, Fig. 16) to prove 
that in Torpedo a portion of the girdle is included in the first part of the 
skeleton which becomes evident. -In this they are probably correct, but 
had they observed a little more closely they would have noticed that this 
portion of the anlage of the skeleton is not at all girdle-like in form but 
is merely an inward extension of the primary basal, while the real girdle 
or arch appears later growing out ventrally and dorsally from this region. 
The apparent contradiction in the development of the pectoral fin 
girdle earlier or later than the more distal part of the fin skeleton is 
entirely cleared up when we go back to the previous stage in the forma- 
tion of the skeleton. Here, as we have already seen, the concentration 
of skeletogenous mesenchyme progresses uniformly, as in all fins, from 
the ectoderm inward and forms the most primitive support of the fin- 
fold. Finally, after the muscles and nerves are all in position (the 
trapezius excepted), the fin skeleton appears in procartilage by differen- 
tiation of the mesenchyme in the position which it holds in the adult. 
The whole development of the pectoral and pelvic fins is just what we 
should expect for structures arising as local organs of the body-wall, 
and, except for the trapezial connection. already explained, involve no 
other structures but the muscles, nerves, blood-vessels, and skeletogenous 
tissues of the immediate region in which they arise. 
CONCLUSION. 
It will be seen, in conclusion, that all of the important objections of 
the gill-arch theorists have now been met and answered. Nearly every 
condition observed in the paired fins has been shown to exist also in the 
median fins. The facts used by recent writers in defense of the gill- 
arch theory, viz., abortive muscle-buds, fusion of muscle-buds, migration 
of fins during development, collector nerves, origin of the skeleton as a 
continuous procartilaginous anlage, discrepancy of rays and muscles, and 
post-axial rays, are thus shown to have absolutely no weight in such 
