238 bulletin: museum of comparative zoology. 



structure of the trochlearis to be worthy of trust, or even of serious 

 consideration. 



Before closing my account of the development of the cranial nerves 

 and their chief branches in Squalus, I wish to call attention to a phenom- 

 enon seen in still later stages of development, already noted by me in a 

 former paper ('97, p. 455). It appears to me a matter of considerai:)le 

 morphological importance that the ganglion of the dorsal nerve of van 

 "Wijhe's eighth somite (fourth post-otic) — the ventral root of which 

 forms at this stage the first of the five hypoglossus roots — unites in 

 late stages of development with the ganglion cells near the root of the 

 vagus. KupfFer ('90) was the first to make evident the morphological 

 importance of the clearly marked distinction between dorsal and lateral 

 (epibranchial) ganglia in embryos of Cyclostomata. While in the em- 

 bryos of Selachii there is not such a clearly marked distinction, there 

 nevertheless exist at the roots of the vagus groupings of ganglion cells, 

 or at least of neural-crest cells (quite distinct from the lateral, epi- 

 branchial ganglia of this nerve, the ganglion nodosum), which in my 

 opinion are to be regarded as homologous with the dorsal ganglion of 

 the vagus of Ammocoetes.'^ The evidence of the union of dorsal seg- 

 mental ganglia in the vagus is as follows. During development the 

 continuous neural crest in the occipital and trunk regions of Squalus 

 becomes differentiated into clearly marked ganglia, lying opposite the 

 myotomes and connected by a cellular " dorsal commissure " (Balfour, 

 '81), as far forward in the embryo as van Wijhe's seventh somite. Oppo- 

 site the sixth and seventh somites no distinct ganglia appear ; but instead 

 a wide sheet of cells, lying in close juxtaposition to the extended roots of 

 the nerve, is observable. While in early stages the ganglion of the eighth 

 somite is separated by a considerable interval from the roots of the vagus, 

 in later stages it approaches these, and in embryos of 30 mm. is seen to 

 be in union with them as a well marked ganglionic appendage. In later 

 stages, its fusion appears complete. The ganglion cells do not degen- 

 erate, but send axis-cylinder processes both centripetally and centrifu- 

 gally, the latter forming the posterior of the roots of the vagus nerve. 

 The ganglion of the second hypoglossus root (ninth somite) does not, 

 however, so fuse with the vagus, but is seen in embryos of 50 mm. as a 

 group of cells without nerve relations, so far as I am able to determine, 

 enclosed in the cartilage of the cranium. It apparently disappears in 



1 Tliese are probably the liomologues of the intracranial ganglia of Ganoids 

 (see Allis, '97, p. 747). 



