144 SECTIONAL ADDRESSES 
one below, form respectively the pineal body and the nervous lobe of 
the pituitary. In their development these two bodies show a remarkable 
resemblance. No true nerve cells are found in them, but they contain 
peculiarly modified neuroglia and ependyma cells. Both pineal and 
pituitary present in some animals appearances strongly suggestive of 
glandular function, their acini opening into the third ventricle. In 
reptiles the pineal body and the nervous lobe of the pituitary are com- 
posed of branching tubular structures resembling glands which discharge 
their contents into the cerebrospinal fluid. In birds the nervous lobe 
of the pituitary retains this hollow character. In mammals both organs 
are for the most part solid, though in some the nervous lobe of the 
pituitary retains to a variable extent its original cavity in free communica- 
tion with the third ventricle. The pineal body is said to be missing in 
crocodiles, and the nervous lobe of the pituitary fails to develop in the 
cartilaginous fishes. The receptors for light, the retinze, arise from the 
wall of the diencephalon, and the dorsal or pineal eye, when present, is 
probably a receptor of a similar nature. The pineal eye is but a part of 
a complex system in which glandular structures at its base are to be 
regarded as forerunners of the pineal body of the higher vertebrates. 
A highly specialised form of ependyma, extending from the pineal recess 
over the front of the posterior commissure into the iter, was named by 
Dendy the subcommissural organ. Its function is unknown, but its 
histological, appearance is suggestive of its being a receptor, open to 
stimulus by the cerebrospinal fluid which bathes its surface while passing 
from the third ventricle into the iter. 
The pituitary body is the only one of the diverse structures of the 
diencephalon which receives an accession of epithelium from an outside 
source. This it gets at a very early stage of development by the ingrowth 
of Rathke’s pocket from the buccal epithelium. ‘The pouch or pocket 
retains its original cavity in most mammals as the cleft of the pituitary, 
and its dorsal wall comes into intimate contact with the nervous outgrowth, 
maintaining and even increasing this intimacy during the life of the 
individual. ‘The union of buccal epithelium with the nervous element 
appears indeed to be to some extent a symbiosis. ‘Transplantation of 
the pituitary body in the tadpole is said to be successful only when a 
certain amount of nervous tissue is included with the epithelial. The 
invasion, of the nervous lobe of the pituitary by this epithelium takes place 
normally to a variable extent, and it is not uncommon to find masses of 
these cells attached to the floor of the third ventricle or scattered individually 
in the cerebrospinal fluid in the neighbourhood of the infundibulum. 
The coming together at an early stage of development of the pituitary 
of two hollow processes, the one from the mouth and the other from the 
wall of the diencephalon, has naturally suggested that their union denotes 
an old association between the brain tube and the alimentary canal. 
Julin pointed out the resemblance of the early stages of development of 
the pituitary to that of the subneural gland of the Ascidian larva, a gland 
which is often spoken of as the Ascidian hypophysis. Kupffer regarded 
Rathke’s pocket as the vertebrate representative of an ancient mouth, 
the palzostoma, denoting an ancestral communication between the 
