LIMBS 



103 



formed in all the Fishes proper and in Dipnoans ; these, which 

 may conveniently be distinguished as " pterygiophores" may unite 

 to form a Ixtsipterygium, and in bony Fishes they become ex- 

 tensively ossified : they frequently come into secondary con- 

 nection with the vertebral column (e.g., by means of the so-called 

 " interspinous bones" of Teleostei). The peripheral part of the 

 fin is supported by dermal rays, which may consist of numerous 

 delicate horny fibres (e.g., Marsipobranchs, Elasmobranchs, Cartilag- 

 inous Ganoids, Dipnoans), or of bony rods, entire or jointed, often 

 cleft at the base, and not preformed in cartilage (Teleosts, Bony 

 Ganoids). 



Median fins are also present in the Amphibia, in which they 

 may persist throughout life (e.g., Perennibranchiata), or may only 



BrF 



FIG. 81. 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. 



i>,"dorsal fin-fold ; S, S, lateral folds, which unite together at S 1 to form the 

 ventral fold ; RF, FF, dorsal fins ; SF, tail-fin ; A F, anal fin ; BrF, pectoral 

 fin ; BF, pelvic fin ; An, anus. 



occur in the larval stage and occasionally also during the breeding 

 season (e.g. Newt). They have the form of a continuous in- 

 tegumentary fold extending round the tail and along the back 

 for a greater or less distance, but enclose no skeletal elements. 



Amongst Reptiles, median fins were present in Ichthyosaurus, 

 and these are comparable to the dorsal fins occurring in the 

 Cetacea amongst Mammals : in both cases they must be looked 

 upon as structures acquired secondarily in connection with ao 

 aquatic existence. 



B. Paired Fins or Limbs. 



Embryological researches have shown that lateral fin-folds must 

 have existed in the ancestors of Vertebrates in addition to the 



