chap, ix.] FINS OF FISHES. 361 



the most proximal articulates directly with the ilium 

 (il), and the most distal is, in the male, converted 

 into the clasper. (See page 519.) The outer portion 

 of the integument of the fin is supported by horny 

 fibres (fn). The pectoral fin (Fig. 154; B) is at least 

 twice as large as the pelvic, and is placed horizontally, 

 but the two halves do not approach one another ven- 

 trally ; there are three basal cartilages, called respec- 

 tively (Gegenbaur) protopterygium (pp), meso- (mep), 

 and metapterygium (mp) ; the latter carries most of 

 the cartilaginous rays, and these are divided into a 

 larger number of pieces than the corresponding rays 

 of the pelvic fin ; as with it, the greater part of the fin 

 is supported only by horny fibres. 



According to the observations of Balfour, the 

 paired fins arise as ridge-like thickenings of the epi- 

 blast (see page 33) ; the mesoblast that invades the 

 ridge gives rise to a cartilaginous bar, which, at first, 

 lies parallel to the long axis of the body. On one 

 side (the outer) of this bar a thin plate extends out- 

 wards, and this, by becoming divided, gives rise to 

 the primary fin rays ; this simple condition is essen- 

 tially retained in the pelvic arch; in the pectoral, 

 however, the basal bar becomes rotated outwards, so 

 that it is now only connected by its anterior end with 

 the pectoral arch, and the bar, in place of being the 

 basal portion, now forms the hinder border of the fin ; 

 the plate attached to the bar becomes imperfectly 

 divided into a smaller proximal and a much larger 

 distal piece ; from the edge of each of these, rays are 

 given off; the smaller piece undergoes a second divi- 

 sion, by which we have, at last, the protopterygium 

 (pt) with one ray, and the mesopterygium with 

 a few ; the rest of the rays are attached to the meta- 

 pterygium, or larger distal piece. 



On the supposition that a many-rayed limb of 

 the characters just described is that from which the 



