136 



COMPARATIVE ANATOMY 



A. Unpaired Fins. 



The unpaired, or median fins, which are mainly characteristic 

 of Fishes, arise in the embryo as a ridge of the integument (ecto- 

 derm and mesoderm) extending along the median dorsal line from 

 the anterior part of the trunk backwards to the tail, around the 

 apex of which it is continued forwards for some distance along the 

 ventral side : thus a dorsal, caudal, and ventral portion can be 

 distinguished. In the course of further development, these 

 portions either remain continuous, or else certain parts undergo 

 reduction, so that the ridge only persists in certain regions, where 

 it forms independent dorsal, caudal, and ventral or anal fins (Fig. 



BF An 



FIG. 96. DIAGRAM SHOWING (A) THE UNDIFFERENTIATED CONDITION OF THE 

 PAIRED AND UNPAIRED FINS IN THE EMBRYO, AND (B) THE MANNER IN 

 WHICH THE PERMANENT FINS ARE FORMED FROM THE CONTINUOUS FOLDS. 



AF, anal fin ; An, anus ; BF, pelvic fin ; Bi'F, pectoral fin ; Z>, dorsal fin-fold ; 

 RF, FF, dorsal fins ; SF, tail-fin ; S, S, lateral folds, which unite together 

 at S 1 to form the ventral fold. 



96, A, B) : in these regions muscles and skeletal parts become 

 developed in Fishes. 



These skeletal parts consist of supporting rays of two kinds. 

 In the base of the fin cartilaginous radii, or pteryyiophores, usually 

 segmented (typically into three portions), are formed ; these may 

 unite proximally to form one or more basipterygia, and in bony 

 Fishes they become extensively ossified : they frequently come 

 into secondary connection with the vertebral column. Except in 

 Cyclostomes, the peripheral part of the fin is supported by dermal 

 rays, which may consist of numerous delicate horny fibres (Elas- 

 mobranchs), or of bony rods, entire or jointed, often cleft at the 

 base, and articulating with the pterygiophores, and not preformed 



