186 Origin of the Paired Limbs of Vertebrates 
finds the shifting concerns not only the distal portion but that even so 
deep-seated a structure as the pectoral girdle is involved in the forward 
migration. Fig. 8 shows the relation of the pectoral girdle to the last 
two gill-arches at the time of the first differentiation of the dorsal part of 
the girdle from the mesenchyme into procartilage. Figs. 9 and 10 show 
later conditions in embryos of 35 mm. and 53 mm., respectively. A com- 
parison of these figures shows that at its earliest appearance the girdle 
is removed from the gill region by a space much greater than that which 
separates consecutive gill-arches, and that as development proceeds the 
space is gradually lessened. In the adult Cestracion the girdle and last 
gill-arch have shifted past each other to the extent that a portion of the 
gill-arch lies internal to the girdle. Could any further evidence be de- 
sired to disprove at once the hypotheses of the serial homology of these 
structures and of the origin of the paired fins by migration? One more 
point, however, demands our attention. 
The trapezius muscle-——This muscle, which is derived from the vis- 
ceral musculature and is supplied by viscero-motor nerves, is attached to 
the anterior surface of the scapular portion of the pectoral arch. With 
this sole exception, all the muscles of the fins, paired and unpaired, are 
derived from myotomes and innervated by spinal nerves. This one ex- 
ception, however, has been the occasion of much controversy, for it is 
considered by the adherents of the gill-arch theory to be a relic (“ Die 
alten Relikte des einstmaligen Kopfmuskelversorgung,” Fiirbringer, 02) 
of the former visceral muscle supply of the pectoral arch when it was a 
gill-arch and before it was abstracted from the branchial region (accord- 
ing to the hypothesis) by the action of the spinal muscles (Ftrbringer, 
02; Braus, 04a). From the contrast which we have already drawn be- 
tween the gills and fins it is evident that such an explanation of the 
trapezial connection is not the correct one. But, aside from such infer- 
ence, I think we can prove the case on its own merits. The studies of 
E. Ruge, 02, and Braus, o4a, showed that in Spinax the first anlage of 
the pectoral girdle consists of the ventral portion only, from the region 
of the fin downward, and that the dorsal or scapular portion appears later. 
This I have been able to confirm in Spinax and to determine also for 
Cestracion (Fig. 7). Now, it is important to note that the trapezius 
muscle is attached to the dorsal or scapular part of the girdle while the 
ventral portion lies entirely within the region of somatic ‘muscles. 
Furthermore, at the time when the scapular part of the girdle appears 
the trapezius has not yet grown backward from the branchial region to its 
point of attachment with the girdle, but a little later grows down and 
