314 
HENRY ORR. 
Orientation as to the direction of this section through the 
brain may be easily acquired by comparing it with fig. 18. 
The section 30 h would be perfectly horizontal in the fig. 18. 
Thus it enters the brain at the hinder edge of the anterior 
baud and passes forward at an acute angle to the morpho- 
logically anterior surface of the brain. In this way the fibres 
passing from the region of the lateral bands to the anterior 
commissure are cut obliquely. The relations of these fibres 
to the lateral bands are shown in fig. 32 g. Here it appears 
that they do not bend and run with the lateral bauds, but may 
be traced for some distance, crossing the latter at right angles. 
The anterior commissure is at first undivided and lies next to 
the surface of the brain, but in the latest stage which I have 
examined an internal part has become divided off from the 
superficial part (fig. 18). This internal part I judge to be the 
corpus callosum. 
The growth of the hind-brain, together with its change of 
form, has in this oldest stage brought the cranial nerves of 
this region much nearer together. These conditions are 
illustrated in figs. 27 h and 28 h. The nerve-roots which are 
present form very large ganglia. The common ganglion of 
the seventh and eighth nerves ( n . VIII, VII) lies relatively 
much nearer the root of the fifth nerve ( n . V) than it did at 
the time of its first appearance. The roots of the ninth and 
tenth nerves appear to have fused in a common ganglion 
( n . X, IX). This may be due to the great growth of the 
auditory vesicle pushing the root of the ninth nerve backward. 
I have been unable to find in these stages any traces of the 
third, fourth, and sixth nerves. In the Lizard the third nerve 
developes as soon as the other ventral roots of the nervous 
system ; the sixth nerve developes somewhat later than the 
other cranial nerves, except the fourth, which first appears at 
a stage much later than the present stage of Amblystoma. 
The olfactory nerve ( n . I) is shown in fig. 29 h, entering the 
olfactory sac (N. a.). The course of this nerve from its origin 
in the fore-brain is backwards and downwards. The fibres of 
the optic nerve are also shown in fig. 30 h, entering the brain 
