THE THYROID AND PARATHYROIDS 279 



In some, however, the right and left lobes are completely 

 separate. The colloid does not begin to be formed in mammals 

 till towards the end of foatal life, and sometimes even not till 

 after birth. 



But before going further it will be necessary to give some 

 account of the post-branchial bodies, which are present in all 

 Gnathostomata except Heptanchus and Teleosts. 



In Selachii the organ was first observed by v. Bemmelen, 

 who called it the " suprapericardial body." He describes 

 it as an evagination of the ventral wall of the pharynx behind 

 the last gill-clejt. This becomes separate from the wall of 

 the pharynx, and now consists of a vesicle lined with epithelium 

 cells. It bears some resemblance to the original rudiment 

 of the median thyroid, and soon consists of a mass of separate 

 vesicles. Whether these contain colloid is not known. 



In Amphibia, Reptilia, and Aves, the post-branchial body 

 is formed as an evagination behind the last gill-cleft, some- 

 times, however, only unilaterally. In these classes of animal 

 the vesicles never contain colloid. 



Of peculiar interest are the researches of Maurer upon 

 Echidna. In this animal, as in all vertebrates, the post- 

 branchial body arises behind the fourth gill-cleft, and soon 

 develops into a gland having a structure like that of the thyroid, 

 the important point being that colloid is contained in the 

 vesicles. In Echidna these post-branchial bodies (colloid 

 glands) never unite with the thyroid proper, which is a large 

 gland, with two lobes and a connecting isthmus. 



In other mammals the post-branchial bodies may assume 

 a structure identical with that of the median thyroid. How 

 much, if any, of the fully developed thyroid is derived from 

 the post-branchial bodies still remains an open question. 



G. Development of the Parathyroids and some other 

 Branchial Cleft Organs 



The development of the thymus will be treated hereafter 

 (p. 225), but it will be convenient in this place to .deal with 

 the development of the parathyroids and the thymus nodules 

 found in connection with the thyroid. 



The particular gill-clefts with which we are at present 

 specially concerned are the third and fourth. 



