NOTES ON MEDIAN AND PAIRED FINS OE FISH. 359 



body are growing rapidly ; but they lengthen at a very 

 different rate, and it is to this fact that " concentration " is 

 due. An embryo 19 mm. long has the first dorsal fin 

 muscles extending over some fourteen segments (fig. 1), an 

 embryo 26 mm. long over about ten segments, an embryo 

 28 mm. long over about four and a half segments. Finally, 

 in the adult dog-fish, the base of the muscles occupies about 

 the length of three segments. A minimum of fourteen fin 

 segments has, then, been relatively concentrated into the 

 length of three trunk segments. 



During conceaitration the hinder limit of the dorsal fins 

 remains approximately at the same place — about the level of 

 the forty-second and fifty-sixth ganglion respectively. The 

 two dorsal fins retain their relative distance during the whole 

 of development ; the second dorsal is always fourteen segments 

 behind the first. Concentration takes place very much more 

 at the front tlian at the hind end of the fins. Their anterior 

 edge, then, moves backwards relatively to the body. Each 

 fin, as a whole, remains throughout in the same position. 



In the case of the paired fins concentration takes place in 

 exactly the same way. But here it is not so pronounced, and 

 the apparent motion of the fins is less. 



The Fusion of the Muscle-Buds. 



It has been stated above that in the normally developed 

 region of the fins each muscle-bud gives rise to one radial 

 muscle. Mollier (24) describes and figures certain strands of 

 cells which at a certain stage unite the bases of the developing 

 radial muscles, suggesting that the adult muscle may con- 

 tain cells derived from several buds (p. 338, above). I find 

 similar strands joining the bases of the muscles in both the 

 paired and the median fins of Scyllium canicula (figs. 6 and 

 19). They are most conspicuous in embryos of about 28 mm. 

 in length. In the early stages of development, when the 

 buds are still of embryonic tissue, no such connecting bands 

 are present. On the other hand, when the radial muscles are 



