THE THYROID AND PARATHYROIDS 277 



of the parathyroid. Again, parathyroids may be found either 

 in the cortex or in the medulla of the thymus gland. When 

 we bear in mind the development of these various organs (see 

 below, pp. 277 and 279), such intimate anatomical connections 

 and occasionally even apparent confusions are not astonishing. 

 In some cases the parathyroids are found to contain colloid 

 vesicles. Figs. 80 and 81 illustrate this very clearly. The 

 drawings represent what topographically is, or, to be more 

 correct, was, parathyroid ; but many parts of the substance 

 are studded with colloid vesicles. 



F. Development of the Thyroid 



The thyroid is the chief of the organs arising from, or in 

 close topographical relation to, the gill-clefts. Other organs 

 in the same group are the thymus, the parathyroids, the 

 post-branchial bodies, 



and the branchial >- end. 



bodies of the Anura. %-*" " v " ^ 



The origin of the ^2^ffi^ 



thyroid is practically ** ^te- ' 



the same throughout ', -/' *^5?fe^ 



vertebrates. It arises 

 from the ventral wall of 

 the pharynx in its an- 

 terior region, as an, ^s^ 



unpaired Outward pro- FIG. 81. A small portion of the parathy- 



iection of epithelium. roid shown in the J f s * two figures. 



. . Section is from a part of the parathyroid. 



AS Stated above, in , The drawing was made with a camera 



AmpMoxuS and Ammo- lucida ; high power. It is seen that the 



. -vesicles, although small, are typically 



C03tes, the Opening into thyroidal in character. 



the buccal alimentary end., endothelium ; c. ves., colloid vesicle. 



tube remains patent, 



and the thyroid appears to be an organ of very ancient 

 origin, which shows relationship to the hypobranchial furrow 

 of Tunicates. In Petromyzon the organ detaches itself com- 

 pletely from its place of origin, the opening becomes closed, 

 and a number of closed vesicles are formed, lined, with 

 cylindrical epithelium, and containing colloid. 



In all vertebrates above the Cyclostomata the unpaired 

 rudiment of the thyroid arises as an evagination of the floor 



