68 THE STRUCTURE OF MAN 



of approximate structural uniformity is finally reached among 

 the Fishes. In the higher types, and especially in Birds and 

 Mammals, the limbs have greatly diverged. In the former, the 

 whole weight of the body is thrown on to the posterior limbs, 

 which are thus purely supporting organs; and the anterior 

 limbs, relieved of their original supporting functions, have 

 become transformed into organs of night. 

 f.d. 



FIG. 48. DIAGRAM ILLUSTRATING THE DEVELOPMENT OF THE FINS OP A FISH. 

 A, To show the first formed and originally continuous lateral (/..) and dorsal (f.d.) 



fin-folds ; f.v. indicates the point where the lateral folds are continued ventrally 



behind the anus (a.). 

 B To show the definitive fins [which owe their independence to the absorption of the 



primarily continuous folds throughout the areas indicated by the dotted lines], d'., d"., 



dorsal fins ; pc., pectoral ; pi., ventral or pelvic tins ; v., anal ; and c., caudal fin. 



An almost equally advanced modification is found in many 

 Mammals, e.g. Man, in whom the anterior limbs have been trans- 

 formed from ambulatory into prehensile organs, the " fore-feet " 

 becoming hands. 



A detailed comparison between the upper and lower limbs of 

 Man will be instituted at the close of this section (infra, p. 91). 



THE PECTORAL (SHOULDER) AND PELVIC (Hip) GIRDLES 

 That the limb-girdles were of later origin than the skeleton 



of the free limbs is rendered probable by the Ontogeny of all 



Vertebrates. 



The following is the course of development in the embryo Shark : 

 A number of originally separate skeletogenous rays (rd., Fig. 49, A), de- 

 velop in the dermal fin-folds l ; and, by fusion at their proximal ends, even 

 before they are at all chondrified, they give origin to a basaljalate (bs). The 

 anterior ends of the basal plates of opposite sides next approximate (*Fig. 49, 

 B), and finally fuse in the middle line, leaving passages for their related 



1 [Great interest attaches to the recent discovery, that in the Palaeozoic Selachian 

 Cladoselache, these rays retained their primary independence in the adult pelvic 

 fin. Cf. Dean, Jour. Morph., vol. ix. p. 87.] 



