24 Paraphysis and the Pineal Region in Necturus Maculatus 
there is a telencephalic plexus, but the plexus of the hemispheres is 
absent or rudimentary (Burckhardt, 3). In amphibia there is a great 
overgrowth of all the plexuses, especially of the diencephalic plexus, 
which here reaches its highest development. In reptilia the plexus of the 
hemispheres is well developed, but the telencephalic plexus is reduced 
in size (Burckhardt, 3), and the diencephalic much more so. In birds 
the plexus of the hemispheres is highly developed, the telencephalic 
plexus practically absent, and the diencephalic plexus, while very similar 
to that of reptilia, approaches nearer to the tela choroidea of higher 
mammalia. 
Osborn first named the supra-commissure and worked out its homol- 
ogies. According to him (30) the urodela are distinguished from the 
anura by the frequent extensive development of this commissure, which 
is large in Amphiuma, smaller in Necturus, and much reduced in Rana. 
It appears in Necturus a little later than the posterior commissure, as 
is usual in most cases, as far as I am aware, except in Ammocoetes, where 
it appears shortly before the posterior commissure (Kupffer, 24, Fig. 5). 
It is found in all the chief types of vertebrates, and is usually smaller 
than the posterior (Minot, 28). It is developed from the diencephalon, 
while the posterior belongs to the cephalic limit of the mid brain. 
CONCLUSIONS. 
1. The paraphysis appears first in an embryo of 12 mm. It is de- 
veloped from the telencephalon immediately cephalad to the velum trans- 
versum as a small diverticulum, which becomes eventually a complicated 
gland with anastomosing tubules. The gland is very vascular, and has a 
sinusoidal circulation. 
2. The epiphysis appears first in an embryo of 9-10 mm., and is de- 
veloped from the diencephalon. It is always attached to the brain by a 
small solid stalk, and the cavity is partially subdivided by incomplete 
septa. 
3. The velum transversum grows at first ventrad and then caudad as 
far as the hind brain, forming in this way the diencephalic portion of 
the choroid plexus. The post-velar arch, which is at first quite extensive, 
is almost entirely absorbed in this extensive growth of the yelum. 
4. The telencephalic plexus arises from the roof of the telencephalon, 
and fills up the depths of the cavity of the third ventricle. The opening 
of the paraphysis is surrounded by these two plexuses. 
5. The plexus of the hemispheres arises at a right angle from the 
telencephalic plexus just cephalad and ventrad to the opening of the 
paraphysis. | 
