FATE OF THE ULTIMOBRANCHIAL BODIES 105 



very irregularly outlined and continuous vacuolar mass to 

 which coarse and loosely arranged cell cords, some of which are 

 vacuolar, are attached. In the extreme caudal portion of the 

 thyroid gland these structures fuse with each other and make up 

 a large portion of its blunt termination. In each embryo a 

 few deeply stained nuclei and a few degenerated nuclei were 

 found. 



Embryo of 75 mm. This is the youngest developmental stage 

 in which colloid is found in the thyroid gland (picro-aceto-formol 

 and hematoxylin and eosin). The follicles containing colloid are 

 not numerous but are quite uniformly distributed throughout 

 the anterior portion of the gland. The ultimobranchial bodies 

 are limited to the posterior two-thirds of the gland lateral to the 

 median plane and bordering the dorsal surface of the gland. 

 They are represented by a continuous area of cell cords w^hich 

 contains no colloid. Within these areas are found small, irregu- 

 larly outlined, and disconnected syncytial masses which contain 

 an unusually large number of small nuclei. These nuclei have 

 the same structure as those found in the cell cords of the thy- 

 roid gland. Many of the cell cords which do not contain colloid 

 are fused to these syncytial masses. Vacuoles are almost en- 

 tirely lacking. A few^ degenerated nuclei are present, found only 

 after prolonged searching. The tripartite complex ends in two 

 blunt processes which have a typical thyroid structure. 



Embryos of 100 mm. The tripartite complex of two embryos 

 deserves notice. In both the colloid is more abundant than in 

 the previous stage. 



Embryo No. 1 (fig. 14). The ultimobranchial bodies are lim- 

 ited to the middle tw^o-fourths of the thyroid gland. The right 

 one lies partially exposed on the dorsal surface of the thyroid 

 lateral to its medial plane. In some places it is composed of 

 coarse and loosely arranged cell cords (U), while in other parts 

 the cell cords merge into a large syncytial mass. In places the 

 connection between it and the thyroid is more intimate than is 

 shown in figure 14. In the syncytial masses the nuclei are on an 

 average a little smaller than those in the cell cords of the thy- 

 roid, but in both their structure is the same. No vacuoles are 



