FATE OF THE ULTIMOBRANCHIAL BODIES 103 



throughout the entire complex. Only a few degenerated nuclei 

 were found in the vacuolar areas. There are realtively more small 

 nuclei present in the vacuolar areas than in the non vacuolar 

 portions and mitoses are more numerous in the latter than in the 

 former areas. 



Embryo of 48 mm. (figs. 11a and lib). Both ultimobran- 

 chial bodies extend anteriorly as far as the thyroid gland. The 

 one on the right side is isolated from the anterior fourth while 

 the one on the left side is isolated from the anterior third of the 

 thyroid gland (fig. 11a, U). Excepting a vacuolar area present 

 in the anterior end of the left one and traces of the lumen caudal 

 to the vacuolar area, the isolated portions of the ultimobran- 

 chial bodies have a structure identical to that of the thyroid 

 gland along which they lie. The right ultimobranchial body 

 near its fusion with the thyroid is quite large (fig. 11a, U). The 

 part fused to the thyroid gland is difficult to follow, yet traces of 

 it may be seen in the form of small vacuolar areas that are pro- 

 miscuously scattered beneath the dorso-lateral margin of the 

 tripartite complex. Some of these areas are sharply outlined 

 while others gradually merge into the compactly arranged cell 

 cords. 



The left ultimobranchial body caudal to its most anterior 

 point of fusion with the thyroid gland is characterized in some 

 places by very irregular vacuolar areas, while in other places by 

 large, closely packed cell cords. In some places also it is only 

 partially fused to the thyroid gland while in other places it is 

 entirely separated from it by connective tissue (fig. 11 b, U.) 

 Traces of the lumen still persist (L). The tripartite complex 

 ends in two large conical processes which have, excepting a small 

 vacuolar area found in each, a typically thyroid structure. Only 

 a few darkly stained nuclei were found. Mitoses throughout 

 the entire complex can be found without much searching. 



Embryo of 53 mm. (fig. 12). Traces of the ultimobranchial 

 bodies are present in the caudal half of the tripartite complex. 

 The anterior end of each is represented by a small irregularly 

 outlined vacuolar area which lies lateral to the median plane 

 just below the dorsal surface of the gland. For some distance 



