FATE OF THE ULTIMOBRANCHIAL BODIES 111 



mobranchial body is not completely transformed into typical thy- 

 roid structures. It lies in the posterior half of the gland and 

 can be traced through a series of 234 sections (2.3 mm.). It is 

 characterized by a small area of tortuous and nonvacuolar 

 syncytial cords free from colloid which is eccentrically located 

 in an area of small follicles (U). The nuclei in the syncytial 

 cords correspond in size and structure to those in the follicular 

 epithelium. A few nuclei in mitotic division are present. No 

 deeply stained or degenerated nuclei are present. 



The right ultimobranchial body extends through a series of 243 

 sections and is found in the middle third of the thyroid gland. 

 It is characterized by an area of small follicles. In both ultimo- 

 branchial bodies from their more central portion toward their 

 periphery the follicles gradually become larger. There is no 

 sharp line of demarcation between these structures and the 

 thyroid gland. 



V. SUMMARY AND DISCUSSION 



By comparing the rate of growth of the ultimobranchial bodies 

 and the thyroid gland, it is seen that a more uniform size ratio is 

 maintained in early than in later developmental stages. During 

 this 'periode d'activite' (Simon) of the ultimobranchial bodies, 

 which extends from an 18 mm. or earlier developmental stage to 

 about a 33 mm. stage, the cephalo-caudal extent of the ultimo- 

 branchial bodies is nearly or entirely equal to that of the thyroid 

 gland. In later stages (33 mm. to full term) in which the ultimo- 

 branchial bodies can be recognized structurally, their cephalo- 

 caudal extent is generally much less . than that of the thyroid 

 gland, which indicates that in later developmental stages the 

 rate of growth of the thyroid exceeds that of the ultimobranchial 

 bodies. In embryos from about 50 mm. in length to full term 

 the ultimobranchial bodies are usually located in the posterior 

 half of the thyroid gland. In a few stages they occur in the 

 middle third or the middle two-fourths of the gland. Simon ('96) 

 claims that during this period of retarded growth of the ultimo- 

 branchial bodies, which he calls the 'periode de survivance,' 

 they undergo degenerative changes which is manifested prin- 



