310 
HENRY ORR. 
and hypophysis ( Hpli .). The morphologically anterior surface 
of the brain has remained in about the same position that it 
occupies in fig. 12 e, but the floor of the hind-brain is bent 
downward and is pressed against the infundibulum. Just an- 
terior to the epiphysis ( Eph .) is a deep fold, extending trans- 
versely across the dorsal wall of the brain, and thus dividing 
off the secondary fore-brain. There is another longitudinal 
and median fold, extending from this transverse fold forward 
to the anterior surface of the brain ; thus dividing the secon- 
dary fore-brain into the two hemispheres. This longitudinal 
fold is not so deep as the transverse fold. Fig. 35 represents a 
section transverse to the long axis of an embryo of the same 
stage as fig. 17 g. This section is behind the deepest extent 
of the median longitudinal fold, but still shows the transverse 
fold. The rudiments of the corpora striata, which are already 
evident at the stage of fig. 13 e, st., are shown again in trans- 
verse section in fig. 35. The corpora striata extend parallel to 
each other on each side of the median line, along the morpho- 
logically anterior surface of the brain, and are limited ventrally 
by the optic groove (o. g., fig. 17 g). Immediately ventral to 
the optic groove is seen the remnant to the anterior fold, con- 
taining a bundle of transverse nerve-fibres, of which a part 
form the optic chiasma ( Ch .). In an exactly median vertical 
section of the brain of an embryo at the stage of fig. 17 g, 
this remnant of the anterior fold would be the thickest portion 
of the brain wall, being about as thick as the lateral walls of 
the medulla. The thickness of the floor of the hind-brain in 
the median line is shown in fig. 24 G, w. H. B. 
Before the embryo of Amblystoma has reached the stage of 
development represented by fig. 17 g, the first development of 
nerve-fibres has taken place in the central nei'vous system. 
The arrangement of these nerve-fibres corresponds very closely 
to the first arrangement of the nerve-fibres in the Lizard, and 
the arrangement seems to be identically the same in Triton 
and Rana. The nerve-fibres in the neural tube of the dorsal 
region first appear as two flat bands of longitudinal fibres, 
lying next the lateral surfaces of the tube. Fig. 34 shows a 
