184 Origin of the Paired Limbs of Vertebrates 
8. The muscles of the median and paired fins are in all respects similar 
in origin while those of the gills are entirely different. 
9. The nerves which supply the paired and unpaired fins are of the 
same sort, all branches of the spinal nerves (that of the trapezius muscle 
excepted) while the gills are innervated entirely by visceral nerves. 
10. The blood-supply of all the fins is the same in character, 7. e., all 
are supplied by those vessels which go to the body-wall of that particular 
region. In the pectoral fin at the earliest stage there is not the slightest 
indication of any other blood connection than that of the brachial artery. 
Fig. 14 shows the origin of this vessel in the manner characteristic of all 
arteries supplying the outer wall of the trunk region, in a young embryo 
of Cestracion, in contrast with the termination of the last efferent 
branchial artery. 
11. Ceratotrichia, or horny fin-rays, are always present in both paired 
and unpaired fins of all sharks, even the most ancient (Dean, 94; Good- 
rich, 03; Osburn, o6b), and are unknown in gills. 
Fin GrrpLes AND GILL-ARCHES. 
The gill-arch theorists still maintain (Fiirbringer, 02; Braus, o4a, 
o4b), as a necessary foundation stone of their theory, that the limb 
girdles are serially homologous with, and in every respect the counterpart 
of, the gill-arches. Let us examine this so-called serial homology. It is 
true that the pectoral girdle does originate close behind the gill region, 
and that in form it is similar, as any structure situated in the body-wall 
as the pectoral girdle is, must of necessity be arciform. But here the 
evidence in favor of the serial homology ceases. Examining the evidence 
on the negative side of the question, we find in the first place that the 
pectoral girdle makes its appearance quite ventral to the gill region. 
Braus’s own figures of Spinax show that the anlage of the girdle is situ- 
ated almost its whole length below the gill region (Braus, oga, Fig. 1, 
Taf. XIII), and the accuracy of this illustration I can attest from my 
own studies on a 20 mm. embryo of Spinax, as well as from my Cestracion 
studies. Fig. 7 shows a camera drawing to the same scale, of a section 
through the fourth gill-arch and one through the pectoral girdle at an 
early stage of the latter before the dorsal portion makes its appearance. 
The dotted lines connect similar points. The contrast is obvious. 
In the second place the more external position of the pectoral girdle 
prevents any homology with the gills. Fig. 7 illustrates this contrast. 
The gill-arch is in contact with the pharyngeal endoderm and is internal 
to the blood system while the pectoral arch comes into contact with the 
