NOTES ON MEDIAN AND PAIRED EINS OF FISH. 361 



The tliirteenth nerve is generally tlie last to contribute to tlie 

 pectoral fin plexus. Occasionally the fourteenth also sends a 

 t.wig-, while sometimes the twelfth is the last of the plexus. 

 The second and third nerves generally send branches which, 

 together with the fourth nerve, pass through a foramen in 

 the girdle to reach the fin muscles. The nerves 4-13 pass 

 behind the girdle. 



About eleven nerves supply the pelvic fin. Of these the 

 last usually belongs to the thirty-fifth segment and the first 

 to the twenty-fifth segment. The first three nerves may form 

 a collector passing through the girdle. The twenty-fourth 

 and twenty-third may also contribute some fibres in front, 

 and the thirty-sixth and thirty-seventh behind. 



The plexus of the first dorsal fin is made of branches from 

 about the twenty-seventh to the forty-third nerves. Very 

 small twigs possibly enter into it from the twenty-sixth and 

 twenty-fifth nerves, but it is probable that these, and perhaps 

 also those of the twenty-seventh and twenty-eighth, are 

 merely sensory. The plexus of the second dorsal spreads 

 from about the forty-fourth to the fifty-seventh nerves. 



In the embryo the first nerve was taken to correspond 

 to the first ganglion. Several small myotomes, some four or 

 five, occur in front of the first ganglion. They appear to be 

 represented in the adult by those small myotomes which lie 

 in front of the first spinal nerve, and are supplied by the 

 spino-occipital nerves. Text figure 2 represents the condition 

 of the fins in the adult if concentration had not taken place. 

 The fins have here been deconcentrated. 



Now, it is only necessary to compare these three diagrams 

 to see that the position of the fins has remained approximately 

 the same throughout development. Concentration, however, 

 has brought about considerable apparent shifting of the 

 pelvic fins, but there is a fixed point in the neighbourhood of 

 nerves 28-30. In the case of the pectoral fin the drawing- 

 back of the anterior margin of the fin has been almost entirely 

 compensated by the drawing forward of the posterior margin, 

 so that in spite of great concentration the position of the fin 



