278 THE DUCTLESS GLANDS 



of the pharynx between the bases of the first and second 

 branchial arches. The structure then separates itself com- 

 pletely from its original point of growth, and appears as a 

 closed vesicle. In fishes the organ remains unpaired, but 

 in Amphibia it divides into two portions whoseTposition differs 

 in different species. 



In the Sauropsida the origin of the thyroid is very similar. 



The original vesicle becomes a compact organ, and in 

 Lacerta becomes developed into a bilobed organ with a median 

 isthmus. In the interior one can for a long time trace a 

 lumen which is the remains of the aperture of the original 

 vesicle. This is lined with cylindrical epithelium. The 

 colloid-containing vesicles do not, however, communicate 

 with a canal. 



In birds the two lobes become quite separate. 



The origin of the thyroid in mammals has been, and still 

 is, a matter of considerable controversy. There can be no 

 doubt about the median rudiment, which arises in the same 

 way as in other vertebrates i.e., as an evagination of the 

 floor of the pharynx between the first and second branchial 

 arches. In the human embryo the evagination is a small 

 pouch beginning to expand laterally in an embryo of 5 milli- 

 metres in length ; in an embryo of 10 millimetres the lateral 

 expansion has much increased, and there is a median duct, 

 the opening of which upon the surface of the tongue corresponds 

 to the foramen caecum ; the duct itself is known as the ductus 

 ihyreo-glossus. The whole structure now consists of a bilateral 

 epithelial vesicle connected by a slender hollow pedicle with 

 the surface of the tongue. The duct persists up to the eighth 

 week, gradually elongating as the thyroid and the tongue 

 separate. The duct usually begins to be obliterated during 

 the fifth week, but sometimes persists. After the fifth week 

 the vesicular portion expands rapidly. 



The development in other mammals does not call for a 

 separate description. 



Many authors describe a lateral rudiment in addition to 

 the median, and this is accepted as the true account by the 

 authors of some textbooks, but this lateral rudiment appears 

 to be that of the post-branchial body (vide infra, p. 279). 



In most mammals the thyroid remains a single organ, 

 consisting, as in man, of two lobes, with a connecting isthmus. 





