O_o 
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. (Pl. 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 » to 210 » 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,’ 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 (Pl. IV., Figs. 12 and 13, 7h.), 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. 
1 Miiller’s Archiv, 1850, p. 504. 
