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 flight. 
Fria. 48.—DIAGRAM ILLUSTRATING THE DEVELOPMENT OF THE FINS OF A FISH. 
A, To show the first formed and originally continuous lateral (f./.) 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 ; pl., ventral or pelvic fins; v., anal; and ¢., 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 (7nfra, 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!; and, by fusion at their proximal ends, even 
before they are at all chondrified, they give origin to a basal plate (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 Paleozoic Selachian 
Cladoselache, these rays retained their primary independence in the adult pelvic 
fin. Of. Dean, Jour. Morph.,-vol. ix. p. 87.] 
