24 DR. A. MILNES MARSHALL. 



"VYe have already sec^u that the neural ridge makes its first 

 appearance in the widest part of the mid brain (fig. 1, m). By 

 the twenty-fourth hour it has grown very considerably at this 

 point {vide fig. 4, m) ; and at the twenty-ninth hour it forms a 

 very prominent outgrowth (fig. S, vi), the neural ridge at this 

 period, as in all the earlier ones, being considerably larger in 

 the mid brain than at any other part of the neural canal. 



Hence we see that, just as in the case of the olfactory nerve, 

 there is a very strong a priori probability in favour of this out- 

 growth developing into one of the cranial nerves ; and since the 

 third nerve is the only nerve that arises from the mid brain in 

 the adult, the probability is very little less that it is the third 

 nerve into which this outgrowth developes. 



Fig. 23 represents a longitudinal section through the head of 

 a ninety-six-hours' chick, passing a little to the left of the 

 median longitudinal plane. A large nerve (III) is seen arising 

 from the base of the mid brain (?« ^), and running downwards 

 and backwards to a point a little posterior to the optic nerve 

 (II) where it terminates in a ganglionic swelling. The nerve is 

 also ganglionic at its base, which is widened, and somewhat 

 conical in shape. 



This nerve, I may state at once, is conclusively shown by its 

 later history to be the third nerve. Its point of origin, its 

 directions, relations, and distribution, at a slightly later period, 

 to certain of the eye muscles prove this absolutely. 



In figs. 17 — 20 we have somewhat different views of this 

 nerve at about the same period — ninety-three hours. The 

 plan in which these sections are taken has been already ex- 

 plained. In fig. 17 the third nerves (III) are seen to lie near 

 the eyes, and well to the inner side of the ophthalmic branch of 

 the fifth nerve (V); to this latter point I shall refer again later 

 on. Fig. 20 passes through the points of origin of the nerves 

 (III). They are seen to arise from the ventral surface of the 

 mid brain (m b), near to the median ventral line, so that the 

 nerves of the two sides are in very close proximity to one another, 

 a very characteristic feature of the adult nerves. 



Longitudinal sections at sixty-seven hours show the nerve 

 (III) in very much the same condition as in fig. 12, but not 

 quite so long. At sixty hours the nerves are more slender, and 

 their roots do not arise so near the median ventral line, so that 

 the nerves of the two .sides are rather wider a})art. 



In younger specimens the nerves are very diflicult to recognise 

 satisfacti)rily. This is due in part to the rapid growtii and 

 shifting relations of the cerebral vesicles, and partly to the fact 

 that the nerve consists in these early stages of cells that are very 

 dillicult to distinguish from the mesoblast cells, which latter are 



