426 DEVELOPMENT OF ELASMOBRANCH FISHES. 



This compound nerve by stage K attains a very complicated 

 structure, and presents several remarkable and unexpected 

 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 repre- 

 sented on the diagram, PI. 17, fig. I. There are present five 

 branches, viz. the glossopharyngeal and four branches of the 

 vagus, arising probably by a considerably greater number of 

 strands from the brain 1 . 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 backwards, and each of them dilates into a 

 ganglionic swelling. They all become again united together 

 by a second thick commissure, 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 ;/./., 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 17, fig. 5. The 

 dorsal commissure is represented in longitudinal section in PI. 1 5, 

 fig. 15 b, com., and in transverse section in PL 17, fig. 2 Vg, com. 

 The lower commissure continued as the intestinal nerve is shewn 

 in PL 15, fig. 15 a, Vg., and as seen in the living embryo in 

 PL 15, figs, i and 2. The ganglia are seen in PL 15, fig. 6, Vg. 

 The junction of the vagus and glossopharyngeal nerves is shewn 

 in PL 15, fig. 10. My observations 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 segmented. It deserves to be 

 noticed that the dorsal commissure has a long stretch, from 



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

 that I have not certainly made out their true number. 



