312 
HENRY ORR. 
stalks. This section is cut transverse to the long axis of the 
embryo. No fibres appear in the region of the infundibulum 
which lies between the anterior band and the anterior edge of 
the above-described continuous ventral commissure. Of the 
brain commissures (not including the anterior band) the pos- 
terior commissure is the first to appear. It developes about 
the time that the ventral commissural system appears. The 
posterior commissure is shown at P. C., figs. 18, 32 g, and 35. 
It crosses the dorsal surface of the brain immediately posterior 
to the epiphysis. Its fibres seem to be not continuous with the 
fibres of the lateral bands, but, as far as they can be traced, 
they cross the course of the lateral bands ; losing themselves, 
however, in the region of the latter. The anterior commissure 
developes relatively much earlier in Amblystoma than in the 
Lizard. It first arises as two lateral symmetrical bundles of 
fibres, passing along the exterior surfaces of the corpora striata 
and intersecting the lateral bands just posterior to tbe optic 
stalks ( A . C., fig. 32 g). This section shows that these fibres 
are not continuous with the fibres of the lateral bands. A 
part of these bundles of fibres crosses the anterior surface of 
the brain a short distance dorsal to the optic groove at the 
point A. C. in figs. 18 and 30 h. The rest of these fibres con- 
tinue on toward the roots of the olfactory nerves, n. 7, fig. 
29 h. A short time after the anterior band has appeared, there 
appears on the morphologically anterior surface of each optic 
stalk a small growth of nerve-fibres, developing as far as can 
be seen, in exactly the same manner as the development of the 
fibres of the lateral longitudinal bands. These optic fibres 
appear at the point n. II, in fig. 32 g (Amblystoma), and are 
shown in fig. 33 (Frog), where they are cut nearly longitudinally. 
The latter section shows that no fibres appear in the posterior 
wall of the optic stalk (op.). Medianly, the optic fibres meet 
and blend with the anterior band ; distally, they pass unbroken 
into the inner surface of the eye-cup (fig. 33). I have not 
followed the later growth of the optic nerve in the Amphibia, 
but I judge from the close similarity between this stage and a 
stage in the Lizard, that the development of the optic nerve 
