1074 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [94] 



jierinaueut rays is fully formed. It is also noteworthy that the most 

 embryonic or solid type of rays, namely, those found in the Diimoi,, are 

 not transverselj^ segmented, apparently for the reason that they are 

 not ossifiedj but retain very much the constitution of the embryonic 

 horny fibres found in fish embryos. The Teleosts have the basal por- 

 tions of the rays unsegmented, especially those proximal portions which 

 are invested by muscles, as is the case with the caudal rays of many 

 forms. The distal portions of the rays with their branches in Teleosts 

 and Sturgeons are segmented and ossified. In young fishes the seg- 

 ments are relatively longer than in the adult, thus indicating that the 

 number of segments of the rays increases with age, as I know from ob- 

 servations made on the development of the caudal rays of Amiurus, in 

 which the segments are at least three times as numerous in very old 

 specimens as in young ones, fifteen days old. This would indicate that 

 the active movement of the fins, as the fish grew larger, caused the 

 semitubular halves of the rays to fracture at more and more points so 

 as to give rise to the increased number of segments. That such fracture 

 does take place may be assumed upon the basis of the investigations of 

 Lotz* and myself. 



Lotz figures the point of fiacture of a ray of the caudal of the young 

 Salmon in which it is shown that the ends of the segments are jagged, 

 as if broken (Fig. 3, PI. Ill), and the membrane is thickened where they 

 come together as if the ends had been bruised by pressing against each 

 other while the whole ray was being beut. In Amiurus, in which I have 

 studied this point, I do not find the broken, jagged ends of the semi- 

 tubular radial membrane so much thickened, as shown by Lotz in the 

 Salmon, but there is a distinctly defined interval between the membra- 

 nous segments, which appear to be held together externally and inter- 

 nally by a thin membrane or sort of ligament, which is very thin and con- 

 sists probably of fibrous connective tissue. These observations have 

 been made upon longitudinal sections of caudal rays, so that there could 

 be no chance of error in my interpretation of the essential facts. They 

 show in the most conclusive manner that mechanical strains upon the 

 membranes when the rays were in use as propelling organs had fractured 

 them at the ])oints described. Moreover, these points of fracture were 

 found "to very often coincide with those of adjacent rays, so that a 

 slightly curved line drawn from the break in a central ray would cut 

 through similar breaks in line with the first in the other rays lying on 

 either side of the former. 



Here we have an instance in which the breaks in a number of adja- 

 cent rays took place concurrentlj' in time and coincidently in position, 

 along a curved line with a generally transverse course across the fin. 

 The probability is that whatever combination of conditions determined 

 the first break in any ray along that line favored the formation of 



* Uebcr cleu Bau dcr Scbwanzwirbelsiiule, &c. 



