carpenter: development of the oculomotor nerve. 165 



Following the article of Dohrn just cited, three investigators published 

 in rapid succession accounts of the development of the oculomotor nerve 

 in selachians. These accounts were remarkable for the fact that they 

 agreed in ascribing to the third nerve an extraordinary origin, the possi- 

 bility of which had apparently never occurred to other investigators. 



Piatt ("91) describes, in embryos of Acanthias vulgaris, a line of nerve 

 cells extending cephalad from the trigeminal ganglion, and soon enlarg- 

 ing into the fundament of a ganglion, which she terms the ciliary. This 

 ganglionic fundament, first meeting an anterior prolongation from the 

 neural crest (which develops into the transitory " thalamic nerve "), finally 

 ends in a mass of 'cells connected with the primary nasal epithelium. 

 This line of cells is later represented by tlie ramus ophthalmicus pro- 

 fundus trigemini. (From the foregoing description I consider it prob- 

 able that Piatt's " ciliary " ganglion is the same as Beard's mesocephalic.) 

 From the inner (median) cells of this ganglion, the oculomotor nerve 

 takes its origin as a cellular proliferation, which grows from the ganglion 

 toward the brain, with wliich it becomes united in the floor of the mid- 

 brain. Two figures are given showing the oculomotor when it consists 

 of but a single cell. The author concludes that the third nerve is, there- 

 fore, primarily sensory, tlie mesocephalic ganglion being at this timo 

 connected with a patch of thickened epitlielium, and no muscle cells 

 having as yet appeared in the walls of the premandibular cavity. 



Mitrophauow ('93) followed with a confirmation of Piatt's account of 

 the origin of the oculomotor. He observed this peculiar development of 

 the nerve in embryos of Raja, Torpedo, and Pristiurus. The ganglion 

 from which the third nerve grows, as a cordon of cells, to tlie brain is 

 plainly the mesocephalic, but the author prefers to call it the ciliary, 

 although he indicates his familiarity with the name mesocephalic by 

 mentioning it several times as a synonym for ciliary. 



Finally, Sedgwick ('94) maintains that nerves do not develop as pro- 

 cesses from central cells, according to the view of His, but arise tlirough 

 the differentiation of a reticular substance already in position. The 

 oculomotor is formed in elasmobranchs as a differentiation of this retic- 

 ulum, resulting from the breaking up of the neural crest, and first appears 

 as a forward projection of nuclei from tlie ciliary (niesoceplialio) ganglion. 

 In the author's words (p. 96), "The third nerve, therefore, presents this 

 interesting and remarkable peculiarity in Scyllium and Acanthias; it 

 grows or is differentiated from the ciliary ganglion to the floor of the 

 mid-brain and not in the opposite direction, as has hitherto been sup- 

 posed." Sedgwick publishes no figures in support of his contention. 



