414 



Nose, Pharynx and its Derivatives in Man 



/ 



histological continuity. On the left side, on the contrary, the connec- 

 tion between the rudiment and the pharynx has disappeared. At a 

 point corresponding to the stalk on the left side there is a short duct 

 pointing toward a similar short duct on the lateral thyroid rudiment. 

 The ventral knob of this rudiment is not only in contact with the 

 median thyroid rudiment at a point corresponding to the one on the 

 right, but a microscopical examination shows that there is actual con- 

 tinuity of the two structures. As in the embryo described just before 

 this, the lateral thyroid rudiments are unlike on the two sides. On 

 the right side the ventral knob is much smaller than the dorsal one, 



while on the left side 

 '^'■■•^' it is the same size or 



a little larger. In 

 both of these models 

 the median thyroid 

 rudiment shows a 

 dorsal enlargement 

 at the right-hand 

 angle of the U. 



In the next stage, 

 represented by em- 

 bryo CXLiy, the 

 median and lateral 

 thyroid rudiments 

 have united on both 

 sides and all con- 

 nection with the 

 pharynx has been 

 lost. They can still 

 be recognized, how- 

 ever (Thyr. 1., Fig. 

 8), as distinct rudiments. Under the microscope, hoAvever, there is 

 structural continuity of the two parts. 



In a later stage, in embryo XLIII, the structures have begun to 

 assume more nearly their adult relations to one another. The lateral 

 rudiments are a part of the median, and a few prominent lobules at the 

 point of their union is all that is left to suggest their existence. The 

 loop of the U is smaller while the limbs have enlarged and show lobu- 

 lated structures suggestive of the condition in the adult gland. 



In embryo XXII (7th week) the shape of the thyroid is almost that 

 of the adult, the small isthmus connecting the two lateral lobes. The 



Thyr.l) 



M-Thym. 



Thyr m - 



Fig. 12. Dorsal view of the dorsal portion of embryo 

 CLXXV, showing the lack of symmetry of the lateral thy- 

 roid rudiments. Oe., oesophagus; Thym.. thymus; Thyr. 

 m., median thyroid rudiment; Thyr. I., lateral thyroid 

 rudiment. 



