100 PROP. MARSHALL AND MR. SPENCER. 



as tending to confirm the correctness of the account, and partly as 

 showing that this curious shifting of the nerve roots, though clearly a 

 change of a secondary nature, must yet have been acquired very early 

 by Vertebrates. 



The later stages of development of the roots of the seventh also pre- 

 sent points of great interest. Fig. 6 represents a transverse section 

 through the roots of the seventh nerve in the same embryo, at stage n, 

 of which fig. 5 shows the roots of the fifth nerve. The seventh nerve is 

 seen to arise on either side by two roots, one (vn a) from the top of the 

 side of the brain at the junction of the thickened part with the thin roof, 

 while the other (vn /3) arises about half way down the side of the brain. 

 Between the two roots the nerve is in contact with the brain, but 

 apparently not connected with it. We have traced the intermediate steps 

 between figs. 3 and 6, and find that the upper root (vn a) of fig. 6 is the 

 original dorsal or primary root, and the lower one (vn/3) the secondary 

 root of fig. 3. In other words, there is an important difference between 

 the fifth and the seventh nerves, inasmuch as in the former the primary 

 root is lost and the secondary alone retained, whilst in the latter both primary 

 and secondary roots are retained up to stage n, and indeed, as we shcdl see 

 immediately, throughout life. The diffei'ence between the roots of the 

 fifth and seventh nerves just noticed does not occur in the case of the 

 chick, in which the primary root of the seventh is lost as completely 

 as is that of the fifth in Elasmobranchs. 1 



This shifting of the roots of origin and acquiring of a secondary 

 connection with the sides of the brain is not confined to the cranial 

 nerves. It has already been shown to occur in the posterior roots of 

 the spinal nerves of the chick, 2 and occurs also in the posterior roots 

 of the spinal nerves of Elasmobranchs. It is a point of much interest 

 to note that the seventh nerve, in the retention of its primary as well 

 as its secondary root, is not only more primitive than the fifth, but more 

 primitive even than the spinal nerves. 



The condition of the roots of the seventh at stage o is shown in the 

 left-hand side of fig. 9, representing half of a transverse section through 

 the hind-brain and roots of origin of this nerve. The two roots, the 

 primary (vii a) and the secondary (vii /3), are even more distinct than 

 at the earlier stages. The primary root (vii a) arises as before from 



1 Marshall, • Quart. Journ. Micr. So.,' Jan., 1878, pp. 24 and 25. 



2 Marshall, "On the Early Stages of Development of the Nerves in Birds," 'Journal of 

 Anatomy,' vol si., 1877." 



