FATE OF THE ULTIMOBRANCHIAL BODIES 107 



extends farther caudally and not so far anteriorly as the right 

 one. These areas which are free from colloid correspond 

 favorably in position to that of the ultimobranchial bodies in 

 some earlier developmental stages. 



Embryos of 125 mm. The ultimobranchial bodies in two 

 embryos of this developmental stage were examined. 



Embryo No. 1 (fig. 15). The ultimobranchial body on the 

 right side lies in the middle two-fourths of the thyroid gland. 

 Its anterior end is deeply imbedded beneath the dorsal surface 

 of the thyroid and is composed of a very irregularly outlined and 

 vacuolar syncytial mass in which the nuclei have about the same 

 size as those in the follicular epithelium of the thyroid. A few 

 pale nuclei are present. Cell cords, some of which are coarse 

 and vacuolar, lead from the vacuolar area and are fused with the 

 surrounding thja-oid structures. Slightly farther caudal it 

 reaches the free surface on the ventro-lateral side of the gland 

 and is composed of a loose network of cell cords some of which are 

 vacuolar. From this place it gradually occupies a more dorsal 

 position in the thyroid gland and is composed of closely packed 

 cell cords, having a structure similar to that of the thyroid gland 

 just before the appearance of colloid. Its more caudal portion 

 reaches the free surface of the thyroid gland on its dorsal aspect 

 (U) and contains a large cyst (C). 



The ultimobranchial body on the left side is represented by a. 

 series of six small disconnected, and irregularly outlined syncy- 

 tial masses which lie just lateral to the mesial plane of the gland. 

 These areas are more or less vacuolar and do not contain any 

 colloid. The thyroid ends in a single process throughout which 

 the colloid is cjuite uniformly distributed. 



Embryo No. 2 (figs. 16 a and 1Gb). The tripartite complex of 

 this embryo is of interest in that the ultimobranchial bodies are 

 only partially imbedded in the thyroid gland. The ultimobran- 

 chial body on the right side lies along the lateral margin of about 

 the middle two-fourths of the thyroid gland to which it is fused. 

 It is fusiform in shape, with its greatest diameter about midway 

 between its ends (fig. 16 a, U). The free portion along its entire 

 extent is composed of syncytial masses and coarse and tortuous 

 cell cords in both of which are found cystoid follicles which are 



