162 bulletin: museum of comparative zoology. 



glion of the second and third branches of the trigeminus. N~ot until the 

 next stage J, does the oculomotor appear ; it then arises by a broad proxi- 

 mal end from the mid-brain. Passing ventrad the third nerve crosses 

 the ophthalmic branch of the fifth on its median side at the level of the 

 mesocephalic ganglion, to which it becomes closely applied ; but, accord- 

 ing to van Wijhe's view, only a close contact, not an actual union, 

 occurs. Later, the oculomotor nerve and the mesocephalic ganglion draw 

 away from each other, although a slender communicating nerve con- 

 tinues to connect the two. 



But van Wijhe's most important contribution to the subject was the 

 discovery of a second ganglion, which, in stage 0, appears as a dumb- 

 bell-shaped mass of cells placed on the branch of the third nerve which 

 supplies the ventral oblique muscle, in the position occupied by the 

 ganglion oculomotorii described by Schwalbe ('79) in adult selachians. 

 This ganglion oculomotorii (which is our ciliary) is comparatively remote 

 from the mesocephalic ganglion, and the author emphasizes the lack of 

 connection between the two. On account of its late appearance, and the 

 presence of a small branch from it to the arteria ophthalmica, he considers 

 the ganglion as belonging to the sympathetic system, but gives no ac- 

 count of its actual development. The mesocephalic ganglion he regards 

 as homologous with a spinal ganglion. The origin of the oculomotor 

 from the base of the brain, the time of its appearance, its histological 

 structure, its lack of a true ganglion in early stages, and its crossing, if 

 not union, with a dorsal root distal to the ganglion of the latter, seem 

 to the author to prove that the nerve in question is a purely ventral one. 



To Marshall's account of the comparatively simple development of the 

 sixth nerve van Wijhe added practically nothing. 



Beard ('85) describes the development in elasmobranchs of what he 

 then called the ciliary ganglion, but later termed the mesocephalic. 

 This account was repeated and supplemented in his notable paper of 

 1887, to which reference has been made. In this paper he clears up 

 the existing confusion caused by the various names given to the ganglia 

 developing in connection with the oculomotor nerve and the ophthalmic 

 branch of the trigeminus. The first he shows to be the true ciliary 

 ganglion of the adult, and, therefore, entitled to that name ; and for the 

 second he proposes, as already stated, the name mesocephalic. He 

 describes the way in which, in elasmobranchs, the mesocephalic ganglion, 

 deriving its cells partly from the neural crest and partly from the ecto- 

 derm in the region of a primitive branchial sense organ, gradually re- 

 cedes from the skin, and fuses with the maxillo-mandibular ganglion. 



