118 J. A. BADERTSCHER 



50 mm. long. He however does not believe that this dense 

 cell area is derived from an ultimobranchial body but that it is 

 ■'niir der Ausdruck intensivem Wachstums der ganzen anlage, 

 wiihrend die Differenzierung der neugebildeten Strange mehr 

 oberflachlich stattfindet; die Zellen sind durchwegs typische Thy- 

 reoideazellen." Kingsbury ('14) finds that a human embryo 25 

 mm. long is the last stage in which the ultimobranchial body is 

 clearly outlined. Their position in succeeding stages up to 41 

 mm. is occupied by a ''poorly circumscribed area of denser tis- 

 sue." He is of the opinion that this "inner condensation" 

 ''marks the place of disappearance of the ultimobranchial body 

 although it may also well be as Grosser has stated, a center of 

 growth." He further states that in 41 mm. and later develop- 

 mental stages the "condensation is no longer recognizable." 

 Although he was unable to satisfy himself as to the actual fate 

 of the ultimobranchial bodies, he is of the opinion that they 

 disappear. 



From a study of the material used in this investigation I feel 

 confident that the structure described by Grosser represents an 

 ultimobranchial body. The process of cell cord formation at the 

 periphery of the 'dichtere Zellgruppierung,' as described by him, 

 corresponds favorably to the process of their formation in the 

 ultimobranchial l3odies in pig embryos. Both the 'inner conden- 

 sation' (Kingsbury) and the 'dichtere Zellgruppierung' (Grosser) 

 apparently represent the central core which in the ultimobran- 

 chial bodies of pig embryos is found in a very wide range of de- 

 velopmental stages, even in a full term embryo (fig. 21). It 

 seems that if the 'dichtere Zellgruppierung' represented a pro- 

 liferati\'e center for the thyroid gland one would expect to find a 

 rather large number of mitotic figures in them as an expression 

 of rapid tissue growth. This, however, is not the case. No 

 more nuclei in division are found in these areas than in any 

 other portion of the thyroid gland. 



The stages in which a comparatively early transformation of the 

 greater portion of the ultimobranchial bodies takes place are com- 

 paratively few in ninnl)er. Also there are comparati\'ely few 

 stages before full term in which there are no areas of small follicles 



