BLOOD-VASCULAR SYSTEM, ETC., OF AMIURUS CATUS. 435 



among which it lies. The frame work of the organ consists of loose 

 connective tissue which does not form a limiting membrane, but 

 simply passes over into the like tissue sheathing the adjacent parts 

 and the vessels which it surrounds. (PI. VIII., Fig. 8.) The usual 

 vesicles of the thyroid are scattered throughout this connective 

 tissue, showing a tendency to arrange themselves in short rows. 

 They vary in size from 15 p. to 210 p. in diameter, and are filled 

 with the usual colloid substance. A few, however, contain a granular 

 substance with nuclei, showing nucleoli scattered through it, while 

 others are part filled with the granular and part with the colloid 

 matter. In the preparation from which Fig. 8 was drawn the colloid 

 matter did not completely fill the vesicles which was probably due 

 to the action of the reagents. 



The wall of the vesicle consists of a single layer of columnar 

 epithelium resting on a basement membrane formed from the sur- 

 rounding connective tissue. The epithelium is readily made out in 

 the young fish, but in the gland from which the section is figured it 

 had almost entirely disappeared. A few brown pigment granules 

 were observed. 



In the youngest specimens (15 mm, long) of which I have sections 

 the gland is very small, and the connective tissue is unattached to 

 any of the neighbouring structures. The vesicles are confined to a 

 few spots and form only a single row. 



THE THYMUS GLAND. 



Considerable interest has centred in the question of the existence 

 of a thymus gland in fishes. Following Stannius' description of its 

 position in the haddock, 1 careful search was made for the gland by 

 dissections on adult fishes but without success. It was afterwards 

 observed and figured by Prof. Wright in sections through the head 

 of a young fish (PI. IV., Figs. 12 and 13, Th.), where it is quite a 

 conspicuous object. This spot was again examined in the adult, and 

 a slight thickening discovered upon the inner surface of the lining 

 membrane of the gill-chamber, in most cases presenting the appear- 

 ance of fat-tissue. As, however, it is impossible to define the gland 

 by dissection on the adult, a description will be given from transverse 

 sections of a young fish. 



i Muller's Archly, 185'), p. 504. 



