3i DK. A. MILN'ES MARSHALL. 



nerves, and that of precisely the nature I have been led to suspect 

 on embryological grounds. 



The seventh and auditor?/ nerves. — la my former account I 

 mentioned that both auditory and facial nerves arose from a 

 single root.i This is in accordance with Balfour's account of the 

 development of the nerves in Elasmobranchs, and is confirmed 

 by my later work. The common outgrowth very speedily 

 divides distally into an anterior branch — the facial, and a 

 posterior — the auditory. 



Like all the other nerves the facial and auditory undergo a 

 considerable change in their point of attachment to the brain. 

 At their earliest appearance they are, as simple prominences of 

 the neural ridge, connected with the hind brain at its extreme 

 summit (fig. 10) ; so that the nerves of the two sides are directly 

 continuous with one another across the top of the neural canal. 

 This attachment is preserved for a short time only ; in the later 

 stages the attachment is to the side of the brain, so that the 

 nerves of the two sides are widely separated from one another. 



This change is eff'ected in the same way in all the nerves in 

 which it occurs, and the following description of the several 

 stages will apply alike to the cranial nerves and to the posterior 

 roots of the spinal nerves. 



Fig. 1 1 represents a transverse section through the hind 

 brain of a forty-seven hours^ chick, passing through the middle 

 of the seventh nerve, and through the anterior edge of the 

 thickening auditory epithelium. The nerve (VII) has attained a 

 great size, and is in close contact with the sides of the hind 

 brain for about a third of its circumference. This contact is 

 especially close at the point a, the lowest point at which it occurs. 

 Still, however, the outline of the brain is clearly and uniformly 

 defined, and the attachment of the nerve is still to the extreme 

 ■summit of the brain only, though that attachment has become 

 very much more slender than it was in the earlier stages. 



Fig. 12 is a transverse section through the same region in a 

 fifty hours' chick. The section passes on the left side through 

 the auditory pit [and), on the right a little further forward, 

 through the combined seventh and auditory nerve (VII). The 

 hind brain has grown very considerably, and its roof, which was 

 before of some thickness, is now very tiiin indeed. The nerve 

 (VII) is still attached by a very slender process {/j) to the ex- 

 treme summit of the brain, but it has also acquired a secondary 

 connection with the brain at the point {a). 



At fifty-four hours the connection at a is somewhat more 

 marked, while the original connection at b is lost completely ; 



' Loc. cit., p. 509. 



