90 J. A. BADERTSCHER 



A study of the ultimobranchial bodies in a wide range of 

 successively older developmental stages of pig embryos of which 

 the thyroid gland was well fixed, has resulted in bringing to light 

 some interesting and important developmental features of these 

 structures in this mammalian type.^ 



II. HISTORICAL 



A brief historical outline representing in a general way the dif- 

 ferent views in regard to the fate of the ultimobranchial bodies 

 is here given. The works of Verdun ('98) and Grosser ('12) 

 render an extensive bibliography in this article unnecessary. 



Born ('83) claims for the thyroid in the pig a triple origin. In 

 a 21 mm. embryo he finds that the nuclei and cytoplasm of the 

 cells composing the lateral thyroids stain more intensely than 

 the cells of the median thyroid, but in a 37 mm. embryo in which 

 the lateral thyroids have become imbedded in and fused with 

 the median thyroid there is no histological difference between these 

 structures and the median thyroid. 



According to Moody ('12) in pig embryos 100 mm. in length 

 no difference is to be observed ''between the central and lateral 

 parts of the gland in vascularity, colloid formation or connective 

 tissue development." He believes that the ultimobranchial 

 bodies contribute to the structural elements of the thyroid 

 gland. 



Simon ('96) claims that in mammals (guineapig, rabbit, cat, 

 calf, sheep, and pig) the lateral thyroids do not actually fuse 

 with the elements of the median thyroid, although they become 

 entirely inbedded in the latter. The lateral thyroids in early 

 developmental stages show signs of growth and further develop- 

 ment. During this period, which he designates the periode d'ac- 

 tivite, the lateral thyroids are broken up into cell cords and cell 



^ No consideration was given to the origin of the structure variously termed 

 'ultimobranchial body,' 'postbranchial body,' 'suprabranchial body,' 'telqbran- 

 chial body,' and 'lateral thyroids.' The morphological value of these terms have 

 been discussed by Greil ('05), Rabl ('09), Kingsbury' ('14), and others. Through- 

 out the descriptive part of this work the term 'ultimobranchial body' will l^e 

 used. 



