74 



Each alveolus is a small roundod of oval closed sac : llie 

 laigest is about 0"07min. 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. Kach 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 Sdlmo* (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 {Tlon. fig. 21) lies internal and slightly 

 posterior to the posterioi- 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 [Th. 4; Th. 5) 

 and inserted into the dorsal border of the operculum. 

 AMien this is dissected oif 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 



■■-;Manrcr, Schilddriise u. Thymus der Teleostier. Morpli. Jahrb.. Bd. xi., 

 pp. l:3U-175, 1880. 



