No. 2.] DEVELOPMENT OF PETROMYZON". 283 



cord (Figs. 44-47, PI. XI), running back to the end of the 

 tail. 



This account of the development of the ninth and tenth nerves 

 is somewhat different from that given by Shipley, but the differ- 

 ence consists for the most part in the fact that he has not ob- 

 served some of the stages. He has overlooked the stage figured 

 in Fig. 36, PI. XI, and says that " the ganglia of the ninth 

 and tenth nerves would seem to arise from a mass of cells split 

 off from the epiblast close behind the ear." Nor docs he seem 

 to have found the second vagus ganglion, or the lateral nerve. 

 " There is no trace of the ramus lateralis even in my oldest 

 larvae." 



Born (10) has mentioned a very interesting and peculiar con- 

 nection between the vagus and the anterior spinal nerves, as 

 occurring in the branchial region. Unfortunately I have been 

 unable to discover how and when this connection takes place, 

 nor could I obtain any observations upon the formation of the 

 hypoglossus. From its appearance and mode of origin in the 

 adult, it would seem to represent the ventral roots of the vagus 

 group ; but this is, of course, uncertain until its developmental 

 history can be worked out. 



The Spinal Nerves. — The development of the spinal nerves 

 has been worked out by Sagemehl (31), and his account is con- 

 firmed by Shipley. My own observations agree, as far as they 

 go, with those of the observers mentioned, and I have nothing 

 to add to their statements. The most important fact to be men- 

 tioned is, that the dorsal roots are primarily all connected 

 by a commissure (Fig. 38, Pi. XI), as was first described 

 by Balfour for the Selachians, and since then observed in many 

 other types. Shipley states that the spinal ganglia are situated 

 opposite to the myotomes, whereas all the sections I have ex- 

 amined show them between the myotomes (Fig. 38). The 

 spinal commissure, as already mentioned, is continuous with that 

 which unites the ganglia of the vagus and glossopharyngeal ; 

 and this would seem to negative, so far at least as the post-audi- 

 tory cranial nerves are concerned, the view maintained by Beard 

 of a radical and fundamental difference between the cerebral and 

 spinal nerves. It is true that the ganglia of the former have a 

 different mode of origin from the latter in that they are partially 

 derived from the external epiblast. But this process, I am in- 



