260 HILL. [Vor 
the lateral position of the anterior vesicle in Teleosts, and even 
more difficult to explain the asymmetry found in Anguis. 
It seems to me that there is some force in this argument as 
indicating that the two vesicles were originally situated side by 
side. If this be granted, it does not necessarily follow that 
they were primitively paired sense organs, each with its own 
nerve supply. An accurate knowledge of their connection 
with the central nervous system, such as might be obtained by 
a study of larval stages, particularly those of Petromyzon, is 
necessary to a solution of this question. 
2. The existence of two epiphyses in Petromyzon and many 
details with regard to them have been made known by the 
work of Ahlborn (83), Scott (87), Owsiannikow (88), Beard 
(9), Gaskell (90) and Shipley (87), nevertheless it may be 
said, in the words used by Scott in speaking of his own work, 
that there is “no observation which shows the mode of origin 
of the second epiphysial vesicle.” The connection of the two 
vesicles with the brain also requires further study. An eluci- 
dation of these two points will do much to clear up the question 
of the primitive condition of the vertebrate epiphysis and until 
it is forthcoming it is useless to speculate on the homology of 
the Cyclostome epiphyses with those of other vertebrates.? 
3. It is of interest to note that the epiphysis of Teleosts 
may be compared in its histological structure to the inner 
layers of the vertebrate retina, the layer of nerve-fibres and 
the layer of nerve-cells. In the vertebrate retina there is an 
internal layer of non-medullated nerve-fibres which lie next the 
vitreous humor and are therefore morphologically superficial to 
the other elements of the retina. These fibres, or most of 
them, originate in a layer of ganglion cells (ganglion nervi 
optici) which are deeper than the nerve-fibres and are not in 
direct continuity with the other cellular elements of the retina. 
In the Teleosts epiphysis there is also a superficial layer of 
non-medullated nerve-fibres which arise from a deeper layer of 
ganglion cells. 
1 Since the completion of this work His has called attention to the importance 
of this subject. Zur allgemeinen Morphologie des Gehirns, Archiv fiir Anatomie 
u. Phys. Ant. Abth. 1892. —S. 346-384. 
