96 . J. A. BADERTSCHER 



types of cells it is distributed in the form of threads and granules. 

 With comparatively low magnification they appear as dark specks 

 among the other nuclei (fig. 2 a). Since these have apparently 

 been regarded by some investigators as degenerating nuclei, they 

 deserve special attention in successively older developmental 

 stages. 



Embryo of 20 mm. (figs. 3 a, 3 b, and 3 c). The uLtimobran- 

 chial bodies are small anteriorly, and extend slightly farther 

 cephalad than the thyroid gland. The one on the left side is 

 separated for a short distance from the extreme anterior part 

 of the thyroid (fig, 3 a). Caudal ward these structures rapidly 

 become larger and form the greater portion of the horns of the 

 crescent shaped tripartite complex. The one on the left side is 

 slightly larger, and eleven sections (5 microns in thickness) 

 longer than the one on the right side and extends as far caudally 

 as does the thyroid gland. The extreme caudal portion of these 

 structures is not fused with the thyroid gland. Remnants of 

 the lumen are present in two places in the anterior third of the 

 left one. 



A feature quite noticeable in the ultimobranchial bodies of 

 this developmental stage is the presence of unusually small 

 nuclei which are found in small groups and promiscuously scat- 

 tered among those of usual size. From their similarity in struc- 

 ture to the larger nuclei they seem to be normal. Deeply stained 

 nuclei, which are somewhat more numerous throughout these 

 structures than in the preceding stage, are also present in these 

 groups. 



It is impossible to determine definitely the exact place of 

 fusion between the ultimobranchial bodies and the thyroid gland. 

 Judging, however, from the uniformity of the distribution of the 

 deeply stained nuclei, from the absence of cell cords along the 

 greater portion of their dorso-mesial free border, from the manner 

 in which they terminate, as stated above, from the absence of 

 blood vessels, and from the distribution of the small nuclei and 

 vacuoles, it seems that the cell masses labeled ultimobranchial 

 bodies in the figures 3 a, 3 b, and 3 c represent exclusively the 

 ultimobranchial bodies. 



