THE THYROID AND PARATHYROIDS 257 



of the single stem for the third and fourth pair of arches. 

 Although richly supplied with blood, it appears of a whitish 

 colour contrasted with the bloodvessels among which it lies. 

 The framework of the organ consists of loose connective tissue 

 which does not form a lining membrane, but simply passes 

 over into the like tissue sheathing the adjacent parts and the 

 vessel which it surrounds. The usual vesicles of the thyroid 

 are scattered throughout this connective tissue, showing a 

 tendency to arrange themselves in short rows. The wall of 

 the vesicle consists of a single layer of columnar epithelium 

 resting on a basement membrane formed from the surrounding 

 connective tissue. The epithelium is readily made out in the 

 young fish, but disappears frequently in the adult. 



On re-examining Amiurus it was found that the connective 

 tissue in which the vesicles lie is very compact and not loose, as 

 described by McKenzie. Further, in none of the slides was the 

 epithelium columnar, and the statement that the cells of the 

 vesicles rest upon a basement membrane could not be verified. 

 In an embryo specimen the vesicles were filled not with colloid, 

 but with some material full of nuclei [epithelium or adenoid 

 tissue (?)]. 



The thyroid of Teleostean fishes is of special interest owing 

 to the fact that goitre (active thyroid hyperplasia) is not 

 uncommon (Marine and Lenhart, Gudernatsch), and is stated 

 to become carcinomatous. The tendency of the growth to 

 spread seems to be connected with the fact that the gland is 

 not circumscribed, has no capsule, and so the gland follicles are 

 distributed over a wide area, and outstanding groups of cells 

 may become fresh foci of tumour formation. 



No parathyroids have jDeenJiound in fishes, but the post- 

 branchial body is developed. 



4. In the Urodela the thyroid occupies a more superficial 

 position than in the Anura. The development has been 

 worked out in Triton, Siredon, and Necturus. The gland 

 arises from a median rudiment, and subsequently becomes 

 paired. 



In Sperlerpes ruber the vesicles are approximately of the 

 same size as in the frog. They do not number more than a 

 dozen. There is less connective tissue than in the frog, and 

 the vesicles are therefore more closely packed together. The 

 colloid contents are of the same character as in vertebrates 



17 



