174 bulletin: museum of compakative zoology. 



which appears to vary in its manner of development not only among 

 animals belonging to different classes of vertebrates, but also among 

 animals belonging to the same class. Even in the same species the 

 accounts of its origin, in one instance, disagree (comp. Beraneck, '84, 

 and Hoffmann, '85). The weight of the evidence, however, seems to jus- 

 tify the following general statements : — 



1. The oculomotor nerve develops after the manner of a ventral 

 spinal nerve from the floor of the mid-brain. Its histogenesis has been 

 described both in accordance with the " process theory," i. e., formation 

 by neuraxons growing out from centrally situated neuroblasts (Neal) ; 

 and in accordance with the " chain theory," i. e., formation by chains of 

 cells which anastomose in the mesenchyme (Dohrn). 



Marshall's theory of a primary connection between the third nerve 

 and the neural crest is not supported by the facts, he himself being 

 unable to trace satisfactorily the intermediate steps in the shifting of 

 the nerve from a dorsal to a ventral position. Kolliker, it is true, finds 

 the nerve in rabbit embryos at first half-way up the side of the neural 

 tube, but his observation in this respect stands alone. Other investi- 

 gators have so convincingly disproved the statements of Piatt, Mitro- 

 phanow and Sedgwick, who hold that the third nerve grows from the 

 mesocephalic ganglion toward the bi-ain, that their erroneous conclusions 

 must be set down to inadequate methods and mistaken interpretations. 



2. The developing oculomotor nerve exhibits throughout its course 

 numerous cells distributed among its fibres. Its proximal extremity is 

 enlarged and crowded with cells. 



With these statements all writers except Ewart and Kolliker agree. 

 Ewart ('90) asserts that in skates at the time of the formation of the 

 ciliary ganglion the oculomotor fibres are free from cells ; and Kolliker 

 ('79) finds no cells connected with the oculomotor in rabbit embryos, 

 except a single peripheral layer which is of mesodermal origin. 



3. The abducent nerve develops after the manner of a ventral spina! 

 nerve, usually by several roots, from the ventral wall of the hind-brain. 

 Numerous cells are associated with the nerve fibres in fishes and birds, 

 but not in reptiles (Beraneck), and probably not in mammals. 



The comparatively few observations on the genesis of the sixth nerve 

 are all in general agreement. 



4. The ciliary ganglion may originate either (a, sympathetic type of 

 development) by the migration of ganglion cells from the mesocephalic 

 ganglion into the oculomotor nerve, either directly or by way of the 

 ophthalmic branch of the trigeminus (Ewart, Hoffmann, Chiarugi) ; or 



