THE SEGMENTAL VALUE OF THE CRANIAL NERVES. 137 



views on these points are of very great interest and importance ; but 

 inasmuch as they involve the descent of Chfetopods and Vertebrates, 

 not from a common segmented ancestral type, but from a common 

 unsegmented type, and also the existence of a group of segmented 

 animals, which " appears now to have perished " without leaving any 

 trace behind, it would clearly be impossible to discuss them here in 

 full. His theory that the vertebrate fore-brain is the homologue of the 

 supra-oesophageal ganglia of Arthropods and Cha?otopods is, however, 

 to my mind open to very serious objections, some of the more weighty 

 of which he has himself mentioned, viz., (1) that there is no actual 

 anatomical or embryological break between the fore-brain and the 

 hinder portion of the central nervous system, such as one might 

 reasonably expect to find on his hypothesis; (2) that the lowest known 

 vertebrate, Amphioxus, instead of lending any support to this view, 

 distinctly contradicts it, the fore-brain being less differentiated from 

 the hinder portion than in any other vertebrate, while " the termina- 

 tion of the notochord immediately behind the fore-brain " — almost 

 the only direct evidence he adduces in favour of the " morphological 

 distinctness " of the fore-brain — again fails completely, the notochord 

 in Ampliioxus, as is well known, extending to the extreme anterior end 

 of the head, some distance beyond the front end of the brain. 



II. The Second or Optic Nerve. — Although, as we have just seen, 

 the statement that the olfactory nerve is rather a part of the brain 

 than a nerve in the strict sense of the word, is found on examination 

 not to hold good, yet, as regards the optic nerve, it is certainly 

 correct ; the mode of development of the optic nerve, which is too 

 well known to require a detailed description here, placing it in this 

 respect in marked contrast to every other nerve in the body. 



From the fore-brain or anterior cerebral vesicle two hollow lateral 

 outgrowths arise — the optic vesicles. These become constricted at 

 their origin from the brain, the constricted portions or optic stalks 

 becoming ultimately the optic nerves. By a process of unequal growth 

 of the different parts, coupled with a direct pushing in of the outer 

 wall by the formation of the lens, each vesicle becomes doubled up 

 on itself, the outer wall being pushed back into the inner, and so 

 giving rise to the double-walled "optic cup" or secondary optic 

 vesicle. 

 I \ This mode of development, which, with secondary modifications, 



