508 THE CHEIROPTERYGIUM. 



These investigations might be regarded as tending to shew that the 

 basipterygium of Elasmobranchii is not represented in Teleostei, owing to 

 the fin rays not having united into a continuous basal bar, but the obser- 

 vations are not sufficiently complete to admit of this conclusion being 

 founded npon them with any certainty. 



The cheiropterygium. 



Observations on the early development of the pentadactyloid 

 limbs of the higher Vertebrata are comparatively scanty. 



The limbs arise as simple outgrowths of the sides of the body, 

 formed both of epiblast and mesoblast. In the Amniota, at all 

 events, they are processes of a special longitudinal ridge known as 

 the Wolffian ridge. In the Amniota they also bear at their ex- 

 tremity a thickened cap of epiblast, which may be compared with 

 the epiblastic fold at the apex of the Elasmobranch fin. 



Both limbs have at first a precisely similar position, both being 

 directed backwards and being parallel to the surface of the body. 



In the Urodela (Gotte) the ulnar and fibular sides are primitively 

 dorsal, and the radial and tibial ventral : in Mammalia however 

 Kolliker states that the radial and tibial edges are from the first 

 anterior. 



The exact changes of position undergone by the limbs in the 

 course of development are not fully understood. To suit a terres- 

 trial mode of life the flexures of the two limbs become gradually 

 more and more opposite, till in Mammalia the corresponding joints 

 of the two limbs are turned in completely opposite directions. 



Within the mesoblast of the limbs a continuous blastema becomes 

 formed, which constitutes the first trace of the skeleton of the limb. 

 The corresponding elements of the two limbs, viz. the humerus and 

 femur, radius and tibia, ulna and fibula, carpal and tarsal bones, 

 metacarpals and metatarsals, and digits, become differentiated within 

 this, by the conversion of definite regions into cartilage, which may 

 either be completely distinct or be at first united. These cartilagi- 

 nous elements subsequently ossify. 



The later development of the parts, more especially of the carpus and 

 tarsus, has been made the subject of considerable study ; and important 

 results have been thereby obtained as to the homology of the various 

 carpal and tarsal bones throughout the Vertebrata ; but this subject is too 

 special to be treated of here. The early development, including the suc- 

 cession of the growth of the different parts, and the extent of continuity 

 primitively obtaining between them, has on the other hand been but little 

 investigated ; recently however the development of the limbs in the Uro- 

 dela has been worked out in this way by two anatomists, Gotte (No. 482) 

 and Strasser (No. 487), and their results, though not on all points in com- 

 plete harmony, are of considerable interest, more especially in their bearing 

 on the derivation of the pentadactyloid limb from the piscine fin. Till 

 however further investigations of the same nature have been made upon 

 other types, the conclusions to be drawn from Gotte and Strassei-'s observa- 



