DEVELOPMENT OF ELASMOBRAXCH FISHES. 203 



This compound nerve by stage K attains a very com- 

 plicated structure, and presents several remarkable and unex- 

 pected features. Since it has not been possible for me 

 completely to elucidate the origin of all its various parts, it 

 will conduce to clearness if I give an account of its structure 

 during stage K or L, and then return to what facts I can 

 mention with reference to its development. Its structure 

 during these stages is represented on the diagram, PI. xvi. 

 fig. 1. There are present five branches, viz. the glosso- 

 pharyngeal and four branches of the vagus, arising probably by 

 a considerably greater number of strands from the brain'. 

 All the strands from the brain are united together by a thin 

 commissure, Vg. com., continuous with the commissure of the 

 posterior roots of the spinal nerves, and from this commissure 

 the five branches are continued obliquely ventralwards and back- 

 wards, and each of them dilates into a ganglionic swelling. 

 They all become again united together by a second thick com- 

 missure, which is continued backwards as the intestinal branch 

 of the vagus nerve Vg. in. The nerves, however, are continued 

 ventralwards each to its respective arch. From the hinder 

 part of the intestinal nerve springs the lateral nerve n.L, at a 

 point whose relations to the branches of the vagus I have not 

 certainly determined. 



The whole nerve-complex formed by the glossopharyngeal 

 and the vagus nerves cannot of course be shewn in any single 

 section. The various roots are shewn in PL XVL fig. 5. The 

 dorsal commissure is represented in longitudinal section in 

 PI. XIV. fig. 15 h, com., and in transverse section in PI. xvi, 

 %• 2 Vg, com. The lower commissure continued as the in- 

 testinal nerve is shewn in PL xiv, fig. 15 a, Vg., and as seen 

 in the living embryo in PL xiv. figs. 1 and 2. The ganglia 

 are seen in PL xiv. fig. 6, Vg. The junction of the vagus and 

 glossopharyngeal nerves is shewn in PL xiv. fig. 10. My obser- 

 vations have not taught me much with reference to the origin of 

 the two commissures, viz. the dorsal one and the one which forms 

 the intestinal branch of the vagus. Very possibly they originate 

 as a single commissure which becomes longitudinally seg- 



1 In the diagram tliere are only five strands represented. This is due to 

 the fact that I have not certainly made out their true number. 



14—2 



