436 PROCEEDINGS OF THE CANADIAN INSTITUTE. 
The gland lies between the epithelium and connective tissue of 
the lining membrane of the gill-chamber in its posterior dorsal por- 
tion. The bulk of the organ lies above the dorso-median angle of 
the chamber extending upwards as a lobe between the trapezius and 
lev. branchiales muscles, and attaining a thickness of 700 yw, or 
eighteen times that of the epithelium, and one-tenth of the vertical 
median diameter of this part of the head. From this thickening 
the gland thins out laterally and medially terminating on a line with 
the floor of the brain-case. Its anterior margin is on a line with 
the third branchial arch, and it terminates behind, slightly in front 
of the transverse process of the supraclavicle. The cavity in the 
glard, shown in the figures, is a mere split in the tissue and without 
a limiting membrane. 
The substance of the gland consist of connective tissue fibres 
mostly parallel to the epithelium and small round nucleated cells not 
larger than 4 4. They are readily distinguished from the epithelial 
cells with which they are in contact by their smaller size and the 
deeper stain imparted to them by various reagents. There are no 
blood spaces and the tissue is homogeneous throughout, except that 
it is looser toward the centre of the gland where the split occurs. 
The gland was secured in the adult by removing the entire mem- 
brane and examined by cutting sections. The greatest thickness 
observed in four specimens was exactly that given above for the 
young fish, and it may be safely stated that in the full grown fish it ~ 
is absolutely smaller. The connective tissue covering it above con- 
tains fat cells, and at places exceeds the gland in thickness. It 
sends processes through to the epithelium at right angles to its sur- 
face. This reticulate connective tissue appears to gradually inerease 
while the cellular elements decrease, and in places undergo fatty 
degeneration. 
The thymus gland in Amiwrws is, therefore, an embryonic struc- 
ture, while the thyroid developes and is functional in the adult 
animal. The former is, no doubt, developed as a diverticulum from 
the epithelium of the branchial cavity as the latter is from the 
mouth. 
It is interesting to find a member of such an old family as the 
Siluroids possessing all those structures (pseudobranchia thyroid 
thymus and head-kidney) which are not, according to our present 
knowledge, constant in their occurrence in fishes, and have been 
