104 PLATT. [Vol. V. 



dorsal wall of the brain, and comes to lie, in horizontal section, 

 above the optic stalk, arising now as a differentiated nerve 

 from the depression between the mesencephalon and thalamen- 

 cephalon, it passes down the sides of the brain, and is lost in the 

 mesoderm above the eye. It finally atrophies at about the time 

 and in the same manner as does the proximal part of the pri- 

 mary trochlearis. Dohrn (No. 8) has described in Torpedo an 

 homologous prolongation of the neural crest. 



12. From the point where the primary trochlearis unites with 

 the trigeminal ganglion a line of nerve cells extends forward, 

 which soon enlarges into the rudiment of the ciliary ganglion, 

 then meets the anterior prolongation from the neural crest, 

 which develops into the nerve that I have called "thalamic," 

 and finally ends in a mass of cells which, arising from the ante- 

 rior margin of the neuropore, have united with the primary 

 nasal epithelium. This line of nerve cells is later represented 

 by the ramus ophthalmicus profundus trigemini. The nerve is 

 therefore primarily a commissural nerve. It unites the original 

 ganglionic cells of the trigeminus, trochlearis, oculomotorius, 

 thalamicus, and olfactorius. 



13. The oculomotorius first appears as a cellular proliferation 

 from the inner cells of the ciliary ganglion. The nerve grows 

 from the ganglion towards the brain, with which it becomes 

 united in the floor of the mid-brain. It is therefore probable 

 that the oculomotorius is primarily sensory ; a conclusion which 

 is strengthened by remembering that at the time when the 

 nerve arises, the ciliary ganglion is connected with a large patch 

 of thickened epithelium, while the walls of the premandibular 

 cavity have not yet developed muscle cells. 



REFERENCES. 



(1) Balfour, F. M. — A Monograph on the Development of the Elasmo- 

 branch Fishes. — London, 1878. 



(2) Comparative Embryology. — London, 1885. 



(3) Beard, J. — The Ciliary or Motoroculi Ganglion and the Ganglion of 

 the ophthalmicus profundus in Sharks. — Anatomischer Anzeiger. Vol. 

 II. 1887. No. 18, 19. 



(4) Beraneck, E. — Recherches sur le de'veloppement des nerfs craniens 

 chez les Le'zards. — Recueil Zoologique Suisse. Vol.1. 1884. 



