THE LIMBS. 3OI 



the ventral fin is a flat, oval paddle, in which we find a 

 feathered or biserial cartilaginous skeleton (Fig. 267). 

 This skeleton consists firstly of a strong, articulated fin-rod 

 or " stem " (Fig. 267, A B), which extends from the base to 

 the tip of the fin, and secondly, of a double row of thin, 

 feathered rays (rr), which are attached to both sides of the 

 central rod, like the pinnee of a pinnate leaf. This primi- 

 tive fin, first recognized by Gegenbaur, and by him called 

 the Arclivpterygiwm, is attached to the spinal column by 

 means of a simple girdle in the shape of a cartilaginous 

 arch. 181 



In some Sharks and Rays, especially when very young, 

 this same primitive fin also occurs in a more or less modified 

 form. But in most Primitive Fishes the fin is already 

 essentially modified, in that the rays on one side of the stem 

 are partly or altogether lost, and are retained only on the 

 other side (Fig. 268). Hence arises the half-feathered, or 

 uniserial fish-fin, inherited by the other fishes from the 

 Selachii (Fig. 269). 



Gegenbaur first showed how the five-fingered leg of 

 Amphibia is developed from this uniserial fin (Fig. 270) and 

 is inherited by three classes of Amniota. In those Dip- 

 neusta which were the ancestors of the Amphibia, the fin rays 

 on the other side of the stem also were gradually degraded 

 in development, and were in a great measure lost (the light- 

 coloured cartilages in Fig. 269). Only the four lowest rays 

 (shaded in Fig. 269) were retained ; and these are the four 

 outer digits of the foot (second to fifth digits). The first, 

 or great digit (toe), on the contrary, originated from the 

 lower part of the fin-rod. From the middle and upper parts 

 of this fin-rod developed the long main stem of the limbs 



