154 PROFESSOR MARSHALL. 



I propose now to consider briefly the leading features exhibited by 

 the eye-muscle nerves individually. 



III. The Third, or Oculomotor Nerve. — Since the third nerve 

 is found to be an independent nerve throughout the vertebrate series, 

 it becomes of interest to inquire whether or not it possesses segmental 

 value. 



Observations by different investigators during the last few years 

 have tended very strongly to support, if, indeed, they may not be 

 said to have established, the claim of the third nerve to rank amono- 

 segmental nerves. Inasmuch as this point has been very fully dis- 

 cussed recently 1 1 do not propose to go over the whole of the evidence 

 here, but shall merely apply, in a somewhat summary manner, the 

 several tests of segmental value in the order given on a previous page. 2 



1. Though the earliest stages of development of the third nerve 

 have not yet been ascertained with precision in any case, yet there is 

 very strong reason for thinking that in the chick, at any rate, the 

 third nerve develops, like the hinder cranial nerves and the posterior 

 roots of the spinal nerves, as an outgrowth from the neural crest on 

 the top of the mid-brain. 3 



2. Inasmuch as, at a rather later, though still early period — about 

 the sixtieth hour in the chick, and stage K of Balfour's nomenclature 

 in the dog-fish — the third nerve arises from the base of the mid-brain, 

 very near the mid-ventral line, it is clear that, if the observations on 

 the earlier stages are correct, the roots must shift downwards at an 

 early period, and to an extent unequalled by any other nerve. 



Kolliker has described the later stages of this shifting, as seen in 

 rabbit embryos, as follows : 4 — In an embryo 12 days 5 hours old, and 

 7 mm. long, the third nerve arose from the mid-brain, not from its 

 ventral surface, but about half-way up its side ; later on it shifts 

 ventralwards, "like the ganglionated cranial nerves and the sensory 

 spinal roots," being found on the ventral surface of the mid-brain in 

 an embryo of the 14th day, and 15 mm. long. 



1 Marshall, "Development of Cranial Nerves in Chick," Quart. Journ, Micros. Science, 

 January 1878, pp. 2S— 27; and "Head Cavities and Associated Nerves of Elasmobranchs," 

 Quart. Journ. Micros. Science, January 1881, pp. 78—83 ; also Schwalbe, Das Ganglion Oculo- 

 motorii. 



» Supra, p. 11. 



8 Cf. Balfour, Comparative Embryology, vol. ii., p. 379. 



1 Kolliker, Entwickelungsgeschichte des Menschen und der hoheren Thieve, Zweite . 

 1879, p. 613. 



