268 THE CHICK. 



along the whole length of the fore-brain, but they do not appear 

 to take any part in the development of the optic nerves. 



III. The motor oculi, or third cranial nerve. The third nerve 

 is the only one which, in the adult bird, arises from the mid-brain. 

 The neural ridges appear first of all on the top of the mid-brain, 

 and early attain a great size in that position (Fig. 118, NA), but 

 it is not yet clear what happens to these ridges in the later 

 stages. It is possible that they take part in the formation of 

 the third nerve, but this has not been proved to be the case. 



The actual date of the first appearance of the third nerve 

 has not been determined. About the middle of the third day 

 it is clearly visible as a nerve of rather large size (Fig. 124, 

 in), arising from the base of the mid-brain, not far from the 

 middle line, and running backwards and downwards towards 

 the hinder border of the eye. 



By the fifth day (Fig. 115, in), the third nerve has the 

 characteristic course of the adult nerve, arising from the floor 

 of the mid-brain and running downwards and backwards im- 

 mediately behind the eye. 



There are strong reasons for regarding the third nerve as 

 corresponding to at any rate a part of a typical cranial nerve, 

 but until its early development is more clearly ascertained it is 

 impossible to speak definitely with regard to it. Its origin 

 from the base of the brain, close to the median plane, its dis- 

 tribution to muscles, and the fact that its root in the early stages 

 (Fig. 115, in) is multiple, have led most investigators to compare 

 it with the ventral root of a spinal nerve rather than with the 

 dorsal root. 



The ciliary ganglion is stated to be formed in the chick in 

 connection with a knob-like thickening of the surface epiblast, 

 similar to the sensory patch of a typical cranial nerve. 



IV. The fourth cranial nerve. The fourth nerve in the 

 adult is peculiar, inasmuch as it is the only nerve which 

 arises from the dorsal surface of the brain, and also, so far as is 

 known, the only nerve which arises from a constriction between 

 two brain vesicles instead of from the middle of a vesicle. 



In a chick embryo of the fifth day the fourth nerve is easily 

 recognised. It is very slender, but has already the course and 



