Mervin T. Sudler 



413 



In the moiel of embryo CIX the median thyroid rudiment is U- 

 shaped with a more or less irregular outline. The right arm, however, 

 is decidedly shorter than the left. The transverse part of the thyroid 

 that runs across the middle line is on the level with the 4th visceral 

 pouch. This part shows the usual network structure formed by cords 

 of solid cells. The arms of the U are crescentic in cross-section with 

 the hollow looking away from the middle line and embracing the 

 thymus rudiment, although the two do not come in contact. The 

 lateral rudiments are still connected with the pharynx. They consist 

 of hollow tubes surmounted by solid expansions. These are unlike on 

 the two sides. On the right side the ventral knob is smaller and less 



p.Th.gl. 



Thyr.m. 



V.P.^(Thyr.l.)-- 



Fig. 11. Ventral view of the dorsal portion of the model of embryo CIX. D. Th. gl., 

 ductus thyreoglossus. Oe., cFSophagus ; Thym., thymus; Thyr. m., median thyroid 

 rudiment. 



prominent than the dorsal, while on the left they are about equal in 

 size and placed on the same level. This lack of bilateral symmetry 

 seems an unimportant thing to note here, but that it is not simply an 

 individual variation is shown by the fact that it occurs in the next 

 stages. 



In the model of embryo CLXXV the median U-shaped rudiment has 

 sunken a little lower than in the preceding stage. The right" limb is 

 still smaller and not as long as the one on the left side. The lateral 

 rudiments show a most interesting lack of bilateral symmetry (see Thyr. 

 1., Fig. 13). On the right side the rudiment is still connected with the 

 pharynx by a small hollow stalk. The ventral knob lies in contact with 

 the middle of the right limb of the median rudiment but there is no 



