Raymond C. Osburn 179 
occurs and remains in the adult. Rather they seem to the present writer 
to represent accidental points of contact of neighboring muscle-buds 
which in their embryonic condition fuse merely because of the juxtapo- 
sition of similar cells, and which may later become separated by the 
growth of intervening connective tissues forcing the muscles farther 
apart, and by the development of the embryonic muscle-cells into fibers.” 
As we have seen, they sometimes remain in contact. That these fusions 
are merely accidents of development is indicated by the fact of their 
sporadic and variable occurrence. They do not occur between the distal 
ends of the muscle-bands where they are farther separated, they are 
most abundant where muscle-buds are crowded together (e. g., anal of 
Cestracion), and least abundant where there is least crowding (e. 4., 
inferior caudal of Cestracion). In the paired fins they are most abund- 
ant near the ends of the series of muscles and they are sometimes entirely 
wanting between the buds at the middle of the fins. Moreover, there is 
no uniformity in the extent of the fused portions. 
DISCREPANCIES BETWEEN MUSCLES AND CARTILAGINOUS FIN Rays. 
Discrepancies between the muscles and cartilaginous fin-rays with re- 
gard to arrangement may be present in unpaired as well as paired fins. 
Braus, o4a, first carefully worked out the discrepancy as it exists in the 
pelvic fin of Spinax, and he at once interpreted it as an argument against 
the origin of the paired fins as metameric structures. That such dis- 
crepancy may exist in the paired fins is probably beyond question. This 
condition, however, it now appears,.is not confined to the paired fins 
and therefore the fact does not bear against the fin-fold theory, since the 
median fins, which show an equal amount of discrepancy, are by the gill- 
arch theory assumed to be strictly metameric in origin. Fig. 15 shows 
this discrepancy in the second dorsal fin of a 53 mm. embryo of Cestra- 
cion. It is in all respects similar to that which Braus figures in the 
pelvic of Spinax (oqa, Taf. XIV), and is of even greater extent. 
If we inquire into the cause of such discrepancy as above indicated I 
am convinced that sufficient reason can be found in the adaptation of the 
fin to meet the greatest mechanical needs. This explanation is given 
weight by the variation shown by different fins in the degree of concord- 
ance they exhibit. In some cases which I have examined,—e. g., the 
pectoral of Raja and Cestracion, the anal and pelvic of Chlamydoselachus, 
“A somewhat analogous process is found in the reopening of a portion of 
the Amphibian blastopore in the formation of the anus. 
