2 Paraphysis and the Pineal Region in Necturus Maculatus 
diencephalon. The epiphysal arch is bounded by two angles, which rep- 
resent the position of the future supra and posterior commissures. ‘The 
velum transversum is a simple infolding of the brain roof, and consists 
of two distinct layers, one caudad and one cephalad, the space between 
them being filled by a loose mesenchymal tissue, which later contains 
numerous blood vessels. This figure is practically identical with Minot’s 
figure of acanthias of the same stage (28, Fig. 1), and is, therefore, of 
great importance in showing the homologies of these parts in elasmo- 
branchs and amphibians. It is probable, as Minot states, that these 
arches occur in most of the vertebrate series. 
The term post-velar arch, introduced by Minot (28), is much better 
for purposes of description than the terms “ zirbelpolster ” of German 
writers, and the “ dorsal sack ” or “ postparaphysis ” of American authors. 
TGS ale 
Fic. 1. Embryo of 8-9 mm. Sagittal section, X 63 diams. 
Fic. 2. Embryo of 10 mm. Harvard Embryological Collection, Sagittal 
Series, No. 269, Section 39, * 63 diams. 
Fig. 2 represents the roof of the diencephalon and telencephalon of 
an embryo of 10 mm. The two layers of the velum are nearer together 
and in the region of the epiphysal arch are seen the first signs of the 
epiphysis, #. This structure is a small rounded diverticulum, which 
arises from the cephalic end of the arch. It is hollow and opens into 
the cavity of the fore brain. 
9 
Fig. 3 is a similar section of an embryo of 12 mm. The velum is a 
trifle longer and the epiphysis a little larger than in the preceding figure. 
Immediately cephalad to the velum a very small evagination in the 
paraphysal arch can be seen, P. ‘This is the first sign of the paraphysis, 
and it appears distinctly later than the epiphysis. The latter overlaps 
its short stalk both caudad and cephalad, and at this stage the stalk is 
still hollow, though its cavity was obliterated in this section. 
