Raymond C. Osburn 185 
tomically, the latter embryologically. The latter author, in particular, 
insists that as good a comparison can be drawn with the gills as with the 
unpaired fins and in the same points, viz. : 
“1. Isoherte Radien. 
“2. Basalia als ontogenetisch einheitliche Anlagen. 
“3. Ausbildung uniserial angeordneter Radien: Monostichopterygium. 
“4. Ausbildung biserial angeordneter Radien: Distichopterygium.” 
It may be true that such arrangements of skeletal parts are to be found 
in the gills as well as in the fins, but even in the skeleton the weight of 
evidence is against such comparison and when we attempt to carry it to 
the other structures all similarity ceases. The following points will 
serve to indicate on what a slender basis rests the comparison of gill 
with fin: 
1. The concentration of mesenchyme cells which gives rise to the gill 
skeleton originates internally in contact with the pharyngeal endoderm 
and spreads outward, while that from which the fin skeleton arises origi- 
nates externally in contact with the ectoderm and develops inward. 
Figs. 1 and 2 show this for the fins. 
2. The paired and unpaired fins arise external to the blood system. 
v. e., to the main blood-vessels, while the gill-arches lie internal to these 
Gigs?) 
3. The paired and unpaired fins are external to the ccelom, the gill- 
arches internal. 
4, It is worthy of note that the structures in the branchial series which 
resemble the fin (the arch excepted) are confined to the hyoid whose 
function as a gill is somewhat degenerated, and the structures are un- 
questionably the result of concentration and reduction. But the hyoid 
arch with its appended structures is most certainly not becoming a fin, 
so of what avail is the comparison ? 
5. As shown elsewhere, the pectoral girdle, though somewhat similar 
in shape and position to the gill-arches, is not in series with them. 
6. The radial structures of the gill are always arranged in a vertical 
series (this is equally true of those of the hyoid arch above mentioned), 
while those of both paired and median fins are always in longitudinal 
series. 
7. The fin-fold, which is the earliest indication of the fin, in all cases 
extends longitudinally, i. e., parallel with the axis of the body. The gill 
membranes arise in a vertical position, at right angles or nearly so to the 
body axis. 
