John Warren 23 
into the hind brain. In Fig. 23, however, this extremity appears as a 
marked tuft in the fourth ventricle. ‘The plexuses of the hemispheres 
arise on either side from the origin of the telencephalic plexus and pass 
into the lateral ventricles, extending nearly to their cephalic extremities. 
In Lacerta vivipara (Francotte, 12, Fig. 24) the post-velar arch has 
beén much compressed from before backward so as to form a deep narrow 
angle. At the apex of the angle the folds of the diencephalic plexus are 
seen. The velum is smooth and apparently is not included in the 
formation of the plexus. In Anguis fragilis (Francotte, 10, Figs. 19 
and 15), the post-velar arch does not seem to be so much compressed, 
and the plexus formation somewhat greater. As he says, however, the 
development of those parts in Lacerta is practically the same as in Anguis. 
According to Burckhardt the telencephalic plexus is much reduced 
in size, consisting merely of small folds, but the plexus of the hemis- 
pheres is well developed (Burckhardt, 3). 
In the turtles Humphrey (20) found that the velum of Chelydra is 
but slightly developed, and no diencephalic plexus is formed. All the 
other plexuses are telencephalic in origin. 
Herrick (15, Fig. 5, Pl. VI), shows in Cistudo a well-developed telen- 
cephalic plexus and a diencephalic plexus represented by many folds in 
the caudal layer of the velum and the post-velar arch. In Chelone im- 
bricata Voelzkow (39, Figs. 19 and 22) shows at first a well marked 
velum and a wide post-velar arch. In later stages the velum and prac- 
tically all the arch are thrown into folds to form the diencephalic plexus. 
The telencephalic plexus is also well developed. In the serpents much 
the same arrangement can be seen. The velum (Leydig, 26, Figs. 
2, 5, and 6) forms a prominent fold, and it and the post-velar arch 
form a very vascular plexus. In the crocodilia (Voelzkow, 39, Figs. 
7, 11, 18, 15), the velum and the post-velar arch are at first well 
marked, but the parts later become so compressed from before back- 
ward that the arch forms a deep acute angle in the depths of which plexus 
foldings are seen. ‘The caudal layers of the velum, however, takes no part 
in the plexus formation. In birds, Dexter (5) found that the velum of 
the fowl is small and the post-velar arch broad at first. This becomes 
compressed so as to form an acute angle much as in the crocodilia. The 
cephalic limb of this angle and all the velum is converted into the choroid 
plexus. In birds the plexus of the hemispheres is very well developed, 
but the telencephalic plexus is practically absent. 
In fishes the plexus development is quite simple, in many cases being 
merely the thin membranous roof of the third ventricle; in others, how- 
ever, this is much folded and vascular (Sorensen, 35). In certain forms 
