152 JULIA WORTHINGTON. 
the thickening of the dorsal wall of the infundibular process 
and of the caudal side of its stalk, already described (fig. 13). 
From such a thickening the saccus arises in higher forms 
(Edinger and Hall, 1899), and the slight evagination photo- 
‘graphed in fig. 13 is probably its rudiment. The vascular 
element is also found. The vertebralis impar artery that 
runs cephalad ventral to the notochord divides at the level at 
which the fibres of the vagus leave the brain into the right 
and left intra-cranial arteries. These arteries penetrate into 
the membranous cranium, and run cephalad in its ventral 
wall (figs. 10 and 11). At the level of exit of the motor 
fibres of the trigeminus each of these arteries gives off a 
large branch that divides immediately into three. The first 
of these branches, entering the dura, is directed backward, 
ventral to the medulla, and supplies the caudal end of that 
section of the brain. The second runs dorsad, entering the 
medulla at once, and running on into the cerebellum. The 
third runs forward, first in the membranous capsule, then 
in the dura, until the anterior end of the medulla is 
reached. It then turns sharply mediad, and forms, with 
its fellow of the opposite side, a vascular plexus that 
lies in the cleft between the anterior end of the medulla 
and the posterior end of the ’tween brain (fig. 13). This 
plexus sends arteries into the medulla, mid brain, and 
’*tween brain. Anterior to the foramen of the trigeminus 
the intra-cranial arteries give off a large branch that leaves 
the cranium, but they themselves continue forward in the 
cranial wall, supplying the fore part of the brain. Referring 
again to fig. 18, and noting again the evagination and 
thickening of the dorsal wall of the infundibular process, it 
is very easy to imagine an increase of this evagination that 
would eventually find its way into the cranial cavity through 
the foramen of the stalk. If it contimued to grow it would 
be forced caudad after entering the cranium, the stalk 
blocking its passage cephalad, and would come into con- 
nection with, and finally envelop, the vascular plexus. This 
