FATE OF THE ULTIMOBRANCHIAL BODIES 119 



(ultimobranchial bodies) present. Judging, therefore, from the 

 so variable developmental behavior of the ultimobranchial 

 bodies it seems that the 175 mm. stage referred to above is one 

 in which the ultimobranchial bodies underwent an early trans- 

 formation into typical thyroid structures. 



The portion of the structural elements of the thyroid gland at 

 birth derived from the ultimobranchial bodies is small in compari- 

 son to the part derived from the median thyroid anlage. Owing 

 to the variable de\'elopmental behavior of the former structures 

 the comparative proportion contributed by them and the median 

 thyroid anlage undoubtedly varies in different embryos. Figures 

 22 a, 22 b, and 22 c are diagrammatic representations of the 

 portions derived from the median thyroid anlage and the ultimo- 

 branchial bodies in No. 2 of the 270 mm. (full term) embryos. 



In the posterior portion of the right ultimobranchial body in 

 No. 1 of the 125 mm. embryos is a cyst which extends through 

 a series of sixty-seven sections (10 microns in thickness). It is 

 lined with cuboidal epithelium the cytoplasm of which stained 

 only very faintly. In one place in its lumen an isolated group 

 of cells is found. The nature of its formation is unknown to 

 me. According to Simon ('96) the formation of cysts in these 

 structures is a regular occurrence during their 'periode de sur- 

 vivance' in all animals examined by him, excepting in the pig 

 in which they occurred in five out of eleven specimens. Since 

 cyst formation occurred in only one specimen out of those I 

 studied, it seems to be an exceptional developmental feature in 

 the pig. 



VI. CONCLUSIONS 



1. The ultimobranchial bodies in the pig participate in the 

 formation of thyroid follicles. However, the portion of the gland 

 in full term embryos that is derived from these structures is 

 small in comparison with the part derived from the median 

 thyroid anlage. 



2. The cephalo-caudal extent of the ultimobranchial bodies is 

 equal to or nearly equal to that of the thyroid gland in embryos 

 up to about 33 mm. in length. From this stage on to full term 



