PRIMITIVE SEGMENTATION OF THE VERTEBRATE BRAIN. 469 
structure of Amblystoma may be explained, as McClure has 
suggested, by the departure of recent Amphibia from the main 
Vertebrate line, and the potent retarding and formative influ- 
ence of a large amount of food yolk ; but, as I shall have occa- 
sion to note, it seems more rational to ascribe it, on purely 
phyletic grounds, to variation in position of the sixth nerve 
itself. In regard to the other segments and nerves of the 
hind-brain, I have seen nothing which would lead me to doubt 
the observations already made. 
Béraneck has pointed out that certain of the hind-brain 
segments are in direct relation to certain corresponding 
nerves. Orr has derived the fifth, sixth (seventh, eighth), 
ninth, and tenth nerves by ganglionic cell masses from the 
dorsal surfaces and crests of the fifth, sixth, seventh, ninth, 
and tenth neuromeres respectively, and the sixth at a later 
period from the ventral surface of the sixth neuromere of the 
hind-brain, no nerve being given off from the eighth neuro- 
mere proper, the space adjoining it being occupied by the 
auditory vesicle. McClure has been able to confirm these 
points, with the exception of the point of origin of the abdu- 
cens nerve. This he has been unable to locate exactly, but 
has placed it approximately between the fifth and seventh 
neuromeres. I have been able to observe in young Ambly- 
stoma embryos that the sixth nerve arises by a proliferation 
of cells from the base of the brain, ventral to and slightly in 
front of the root of origin of the seventh and eighth, though 
it has no connection with it (see figs. 11 and 12). In the 
Cod, which, as I have said, shows six hind-brain neuromeres, I 
have not been able to satisfy myself in regard to this nerve, 
but at least am positive that its origin is ventral to and much 
in front of the seventh and eighth. In other words, I am 
inclined to think that Orr is correct in placing its origin at 
the ventral surface of the sixth neuromere in the Lizard, and 
that it has such origin in all species in which six neuromeres 
are present in the hind brain, 1.e. forms in which the nerve 
has retained more or less perfectly its primitive character ; and 
that its deviation from this position and gradual shifting 
