SEA-FISHERIES LABORATORY. 219 



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 Hem. 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 differentia- 

 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, like 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 (SjjL fig. 21) lies on the internal surface 

 of the left lobe of the liver, usually wedged in between 

 * Maurer, Loc. cit. 



