HEAD CAVITIES AND NERVES OF ELASMOBRANCHS, 93 
does to a posterior root, I see no reason to doubt that this nerve 
(the sixth) is to be considered as the anterior root of the 
seventh. 
The sole point of any importance in which the sixth differs 
from a typical anterior root is that it does not join with its poste- 
rior root, the seventh. I do not think, however, that this point 
can be regarded as of sufficient importance to render the com- 
parison invalid. 
It becomes now a matter of much interest to determine, if 
possible, whether anterior roots are present in the other cranial 
segmental nerves. Such roots have been described by Gegenbaur 
in the adult Hewxanchus,’ by Jackson and Clarke in the adult 
Lichinorhinus,* by Balfour? (who, however, regards them as be- 
longing really to the spinal and not the cranial nerves) in both 
adult and embryo examples of Scy/diwm, by myself in the embryo 
chick about the end of the fourth day,* and by others in different 
adult Vertebrates. 
Are there any traces of such roots in the third and fifth 
nerves? In answer to this question I would suggest that the 
small anterior non-ganglionic roots, which I have described as 
appearing in both the third and fifth nerves during stage L, are 
really morphologically equivalent to anterior spinal roots. These 
roots closely agree with the roots of the sixth nerve in appear- 
ance; they are slender, non-ganglionic, multiple, arranged in an 
antero-posterior series, and appear later than the original gan- 
glionic roots. The exact time of their appearance 1 have not 
determined with accuracy, but it is certainly notewortliy that the 
earliest period at which I[ have noticed these additional roots of 
the third and fifth nerves is also that at which I have first 
detected the sixth nerve, viz. stage L. 
In the fact that their anterior roots are situated further forward 
than their posterior roots, the third and fifth nerves resemble 
the spinal nerves,’ but differ from the seventh, whose anterior 
root (the sixth) we have seen to be situated a little behind the 
root of the seventh. This point is probably connected with the 
complete independence retained by the sixth nerve, and alluded 
to above. 
The fact that the anterior and posterior roots of the third 
nerve arise at the same vertical level, and not one below the 
other, is, I think, fully explained by the great change in posi- 
tion which the posterior roots undergo, these posterior roots 
1 ¢ Jenaische Zeitschrift,’ vol. vi. 
? «Journal of Anatomy and Physiology,’ vol. x. 
3 Op. cit., p. 205. 
4 Loe. cit., p. 38. 
° Balfour, op. cit., p. 160, and plate xvi, fig. 1. 
