74 
Each alveolus is a small rounded or oval closed sac; the 
largest is about O°‘O7mm. in diameter, but they vary in 
diameter within wide limits. The wall is made up of a 
single layer of columnar cells outside which a delicate 
sheath may often be distinguished. Hach alveolus is filled 
with the colloidal substance characteristic of the thyroid, 
which usually stains slightly with eosin. It may be con- 
tracted away from the wall in section, but this appearance 
is most probably artificial and in life the alveolus is un- 
doubtedly filled. 
_ The thyroid originates in Salmo* (and probably in all 
Teleosts) as a median evagination of the ventral pharyn- 
geal epithelium which has no connection with the gill 
clefts. This evagination forms a little vesicle, the cavity 
of which at first communicates with that of the pharynx 
by a tubular stalk. Later on the stalk becomes solid and 
the vesicle separates entirely from the pharyngeal wall. 
It then shifts backwards towards the heart, and its wall 
begins to form hollow buds, which later on separate and 
become closed. These persist as the definitive thyroid 
alveoli. Paired rudiments do not, as in higher verte- 
brates, contribute to the formation of the adult gland. 
The Thymus (Zim. fig. 21) lies internal and slightly 
posterior to the posterior and dorsal corner of the oper- 
culum. On dissecting away the skin in this region a sheet 
of muscle fibres is seen originating at the cranial ridge 
connecting the 4th and 5th tuberosities (7'6. 4; Jb. 5) 
and inserted into the dorsal border of the opereulum. 
When this is dissected off the thymus is seen lying under- 
neath and immediately in front of the supra-clavicle 
(S.Cl. fig. 8), between this and a slip of muscle which 
originates in the pterotic at the base of the 4th 
“Maurer, Schilddriise u, Thymus der Teleostier. Morph. Jahrb., Bd. xi., 
pp. 150-175, 1885. 
