EVOLUTION OF THE LIMBS. 303 



Pig. 267. — Bones of pectoral fins of Ceratodus (Aroliipterygium, or 

 bilateral pinnate skeleton) : A B, series of cartilaginous pieces forming the 

 ventral stem of the fin ; rr, rays of the fin. (After Giinther.) 



Fig. 268. — Bones of pectoral fin of an earlier Primitive Pish (Acauthias) . 

 Most of the rays of the medial edge of the fin (B) have disappeared ; only 

 a few (iJ') remain. B R, rays of the lateral edge of fin ; mt, Metap- 

 terygium ; ms, Mezopterygiura ; p, Propterygium. (After Gegenbam-.) 



Pig. 269.' — Bones of pectoral fin of a more recent Primitive Pish, or 

 Selachian. The rays of the medial edge of the fin have entirely dis- 

 appeared. The shaded part on the right is that portion which develops into 

 the five-fingered hand of higher Vertebrates (b, the three basal pieces of 

 the fin ; mt, Metapterygium ; rudiment of the humerus ; ms, Mezoptery- 

 gium; p, Propterygium.). (After Gegenbaur.) 



Pig. 270. — Bones of the fore-limb of an Amphibian : h, upper arm 

 (humerus) ; r, u, lower arm (r, radius ; u, ulna) ; r, c, i, c, u, root-bones of the 

 hand, first row (r, radial ; i, intermediate ; c, central ; ii, ulnary) ; 1, 2, 3, 4, 5, 

 root-bones of the hand, second row. (After Gegenbaur.) 



Pig. 271.— Bones of hand of Gorilla. (After Huxley.) 



Pig. 272. — Bones of human hand, seen from the back. (After H. Meyer.) 



which is so prominent in the higher Vertebrata as the upper 

 arm (or leg) (Fig. 270, r and u) and the lower arm (or 

 leg, h). , 



The many -fingered fish-fins thus gave rise, bj^ a process 

 of gradual reversion and difierentiation, to the five-fingered 

 amphibian foot, which occurs first in the Sozobranchia, and 

 which, from them, has been transmitted on the one hand to 

 Eeptiles, and to Mammals, up to Man, on the other (Fig. 272) 

 Simultaneously with the reduction of the number of the fin- 

 rays to four, a further differentiation affected the fin-stem or 

 rod ; it became transversely divided into the upper and 

 lower arms (or legs), and a modification took place in the 

 girdle, which in the higher Mammals originally consists, 

 both anteriorly and posteriorly, of three bones. The simple 

 arch of the original shoulder-girdle separates, on each side 

 into an upper (dorsal) piece — the shoulder-blade (scapula) 



