75 
tuberosity and is inserted into the upper end of the 
clavicle. It is a flattened glandular mass occasionally 
covered with black pigment spots. In a fish of about 20 
inches in total length it is about 14cm. in length, half 
that in breadth, and about 2mm. in thickness. It lies 
with one edge uppermost. Its artery appears to be a branch 
of the subclavian, its vein opens into the superior jugular. 
It lies immediately external to the roots of the vagus, and 
this association of the gland and nerve appears to be a 
constant one in fish of all sizes from the stage at which 
the structure is definitely formed. Its histological struc- 
ture is that generally characteristic of the thymus gland 
of vertebrata. It consists of small rounded cells closely 
packed together in a narrow-meshed reticulum of connec- 
tive tissue. The cells have large nuclei and attenuated 
cell bodies. The whole gland is surrounded by a loose 
capsule of connective tissue which is continuous with the 
internal reticulum. ‘There is no well marked difterentia- 
tion into cortical and medullary regions except that in 
adult specimens a narrow peripheral zone stains more 
intensely than the rest of the gland. Fatty tissue is little 
developed, and concentric corpuscles are apparently absent. 
The thymus in Salmo* (and probably in all Teleostean 
Fishes) develops, hke the organ in Elasmobranchs, from 
proliferations of the epithelium clothing the dorsal ex- 
tremities of all the gill clefts. These originally separate 
thymus buds fuse together while still in connection with 
the gill clefts, and for a time their cells can be traced 
continuously into the epithelium of the cleft. The whole 
organ then separates from its parent tissue and undergoes 
a backward shifting into its adult position. 
The Spleen (S//. fig. 21) les on the internal surface 
of the left lobe of the liver, usually wedged in between 
* Maurer, Loc. cit. 
