100 J. A. BADERTSCHER 



these stages were not generally distributed throughout the ulti- 

 mobranchial bodies but were found in localized areas. A few 

 darkly stained nuclei can be found in the cell cords of the thy- 

 roid gland during this developmental period. In the thyroid, 

 however, they are never very numerous and in some they are 

 found only after prolonged searching. 



Embryo of 29.5 mm. (figs. 10 a, 10 b, and 10 c). The tripar- 

 tite complex in this embryo is of interest in that a large portion 

 of it is asymmetrical in shape, due to the unequal length of the 

 ultimobranchial bodies. Nearly all of the anterior fourth of the 

 complex is symmetrical and is composed of cell cords of the thy- 

 roid gland only (fig. 10 a). The greater portion of the middle 

 two-fourths of the tripartite complex is characterized by the 

 presence of massive cell cords of the left ultimobranchial body and 

 the entire absence of the right ultimobranchial body (fig. 10 b). 

 Along the posterior fourth of the thyroid gland both ultimo- 

 branchial bodies are present. The left one terminates rather 

 abruptly thirty sections (150 microns) anterior to the caudal end 

 of the thyroid while the right one tapers to a point and extends 

 as far caudally as the thyroid gland. The extreme caudal por- 

 tion of each ultimobranchial body is less broken up into cell 

 cords than is represented in figure 10 c. Small disconnected 

 vacuolar areas are present in the more caudal portion of both 

 branchial bodies. The large cell cords are almost entirely free 

 from vacuoles but are characterized by a comparatively large 

 number of small nuclei. The deeply stained nuclei, which are 

 comparatively few in number, are most confirfed to the ultimo- 

 branchial bodies. Only a few are present in the cell cords of 

 the thyroid gland. Only a very few degenerated nuclei were 

 found. 



In embryos from about 30 mm. in length to stages in which 

 colloid is first present in the folUcles of the thyroid gland (75 

 mm.), the ultimobranchial bodies present a varied appear- 

 ance. They are largely broken up into cell cords and in the 

 progress of development the cell cords of the thyroid gland 

 and usually those of the ultimobranchial bodies have become 

 closely packed together so that a sharp demarcation between the 



