Development of Thyroid 



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Fig. 6a. — Section of thyroid of wild rat. Magnified 250 

 diameters. The vesicles are seen to be distended with 

 colloid and the cells flattened. (Chalmers Watson.) 



large vesicles and without any great accumulation of colloid, to a eland 



with flattened epithelium 

 and vesicles greatly dis- 

 tended by colloid are ex- 

 hibited under the influence 

 of different nutritive sub- 

 stances in the diet : the 

 former conditions being, in 

 the rat, typically seen when 



rthe animals have been fed 

 ' ~*» v . *^T* ,r '**^J upon a mixed food such as 



bread and milk ; the latter 

 when the diet has been 

 composed of lean meat. 

 Prolonged feeding: of rats 

 with lean meat led eventu- 

 ally to a shrinking in size 

 of the vesicles and of the 

 whole organ, these changes 

 being associated with dry- 

 ness of skin, falling off of 

 hair, and other evidences 

 of ill-health ; in carnivora results of this kind were not observed. 



Development. — The thy- 

 roid is originally developed 

 like an ordinary secreting 

 gland by a median outgrowth 

 from the entoderm lining the 

 floor of the pharynx at a 

 level between the first and 

 second branchial pouches 

 (fig. 2). It appears very early 

 and grows backwards as a 

 solid column of cells, bifur- 

 cating at the upper end of 

 the trachea into two lateral 

 portions. The solid column 

 becomes hollowed out into a 

 duct (ductus thyreoglossus), 

 which presently becomes obli- 

 terated, usually completely ; 

 itsoriginal opening is marked 

 throughout life by the fora- 

 men ccecum at the back of 

 the tongue. The lateral parts into which the caudal end divides, branch 



Fig. 6b. —Section of thyroid of another wild rat. Magnified 

 250 diameters. The vesicles contain little or no colloid ; 

 the epithelial cells are columnar. (Chalmers Watson.) 



