1/4 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



of the fin proper would now consist of the basals and radials 

 plus a number of delicate rods of dermal origin, the so-called 

 actinotrichia. From such an archetypal structure the paired 

 fins of all fish-like forms can readily be deduced ; but the transi- 

 tion between this and the limb of the higher vertebrates offers 



Vi 



FIG. 188. A, pectoral, and B, ventral fins 

 of Cladoselache, after Dean, b, basalia ; r, 

 radialia. 



many difficulties, and the interme- 

 diate forms have not yet been 

 found. The most plausible view 

 of the homologies is that which 

 derives the limb of the higher ver- 

 tebrates from one like that of the ganoid or selachian by the 

 loss of most of the basals and radials, a single basal giving rise 

 to the humerus or femur, the proximal portions of a couple of 

 radials forming the ulna and radius (tibia and fibula), while 

 the distal portions of the same radials, with possibly parts of 

 others, producing the distal parts of the limb. The accom- 

 panying diagram illustrates the general outlines of the process, 



FIG. 189. Diagram illustra- 

 ting possible evolution of penta- 

 dactyle limb from the ventral fin 

 of fishes ; the shaded portion 

 represents the persistent parts. 



