98 J. A. BADERTSCHER 



stained nuclei and groups of small nuclei are more numerous 

 than in the preceding stage. 



Embryo of 22 mm. (figs. 6 a and 6 b). The tripartite complex 

 presents extremely varied pictures. Its anterior fourth is com- 

 posed entirely of typical thyroid cell-cords while its caudal por- 

 tion is composed chiefly of the ultimobranchial bodies (fig. 6 b) . 

 The caudal portion of each ultmiobranchial bodj^ is composed 

 of a cell mass of irregular outline in which remnants of the lumen, 

 lined with columnar epithelium, still persist (fig. 6 b, L). An- 

 teriorly, they are largely broken up into coarse cell cords which 

 process marks the beginning of important developmental features 

 in these structures (fig. 6 a, U). Nonvacuolar areas can be 

 found throughout their entire extent. The deeply stained nuclei 

 in the coarse cell cords, in the more central unbroken masses, and 

 in the epithelial buds are more numerous than in any of the pre- 

 ceding stages. They can be quite readily found in the cell 

 cords of the thyroid gland (figs. 6 a and 6 b), but are not nearly 

 as numerous as in the ultimobranchial bodies. No degenerating 

 nuclei, such as pyknotic or fragmented nuclei, were found. 

 Small nuclei, in groups and diffusely scattered, are more numer- 

 ous in the ultimobranchial bodies of this stage than in the pre- 

 vious one. In another 22 mm. embryo the size, shape, and ex- 

 tent of the ultimobranchial body along the thyroid gland is 

 quite similar to that of the 21.5 mm. embryo described above. 



Embryo of 23 mm. (figs. 8 a, 8 b, and 8 c). The ultimobran- 

 chial bodies are small anteriorly and extend slightly farther 

 cephalad than the thyroid gland. For a short distance anteriorly 

 the ultimobranchial bodies and the thyroid are not fused. From 

 their point of fusion with the thyroid they rapidly become larger 

 so that the caudal portion of the tripartite complex is largely 

 composed of the ultimobranchial bodies (figs. 8 a, 8 b, and 8 c, 

 U). The epithelial buds attached to the ultimobranchial bodies 

 are in general not as large as those in the 22 mm. embryo. The 

 darkly stained nuclei and groups of small nuclei are also less 

 numerous than in the preceding stage. Only an occasional 

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



