22 Paraphysis and the Pineal Region in Necturus Maculatus 
described by Kupffer (23) as the “ schaltstiick,” and according to him it 
is best developed in amphibia. Burckhardt (3) maintains that it occurs 
in all vertebrates from Petromyzon up, but according to Kupffer it is 
absent in Accipenser (23, Fig. 19), and it is also wanting in Acanthias 
(Minot, 28, Fig. 10) and in the fowl (Dexter, 5, Fig. 9). 
The velum transversum is probably characteristic of all vertebrates. 
Minot (28). In Petromyzon the velum appears as a small transverse 
fold, and the post-velar arch is well marked. The plexus development 
is, however, very slight. In elasmobranchs the velum of Acanthias 
forms a long, narrow, transverse fold, and the post-velar arch is so small 
that the origin of the velum seems to be close to the supra commissure. 
The caudal layer of the velum is distinctly thinner than the cephalic 
(Minot, 28, Fig. 6). This is also seen in Torpedo (d’Erchia, 7, Fig. 
12), and in Necturus, Fig. 6. The velum later on has the character of 
a choroid plexus, but the plexus of the hemispheres is very rudimentary 
(Minot, 28). In Notidanus Burckhardt (3) shows a long, narrow velum, 
a short post-velar arch, and a small telencephalic plexus. The plexus 
of the hemispheres, however, is absent. 
In Accipenser (Kupffer, 23, Fig. 19) the velum is long, well developed 
and folded to a certain extent, and the post-velar arch is quite extensive. 
In ganoids (Studnicka, 38) the membranous roof of the brain serves as 
the tela choroidea of higher types. In this class of vertebrates according 
to Burckhardt (3) the plexus of the hemispheres is lacking, but the 
telencephalic plexus is well developed, and in teleosts the former is also 
wanting, but the latter present in a reduced form. 
In amphibia all the plexuses are highly developed, and in Necturus 
they are of marked extent (Kingsbury, 21). The velum in Necturus 
appears at first as a transverse fold in the roof of the brain separating 
the diencephalon from the telencephalon. This fold develops at first 
ventrad and then caudad through the mid brain as far as the hind brain. 
This great growth of the velum forms the diencephalic plexus. The 
post-velar arch, which at first is wide and well marked, is practically 
absorbed by the overgrowth of the velum, and a small portion only 
persists in the roof of the diencephalon between the origin of the dien- 
cephalic plexus and the supra commissure, Fig. 17. The telencephalic 
plexus develops from the paraphysal arch immediately cephalad to the 
paraphysis, the opening of which therefore is surrounded by these two 
plexuses. They fill up the cavity of the third ventricle and mid brain, 
and the diencephalic plexus may appear in the hind brain (Osborn, 29). 
This seems to vary in different cases, and in the majority of brains which 
I was able to examine the extremity of this plexus did not actually extend 
