FATE OF THE ULTIMOBRANCHIAL BODIES 109 



The ultimobranchial body on the left side is exposed to the 

 free surface only in a few places. The larger portion lies im- 

 mediately beneath the dorsal border of the thyroid gland. In 

 one of its exposed parts are found cystoid follicles which contain 

 colloid. The imbedded parts have the same structure as the 

 imbedded portion of the right one. No darkly stained nuclei 

 were found in either of the ultimobranchial bodies. 



Embryo of 150 mm. The ultimobranchial bodies are located 

 in the middle and a part of the posterior third of the thyroid 

 gland near its lateral borders. They are represented by areas 

 of typical thyroid structures in which the follicles containing 

 colloid are small and not very numerous. In places they reach 

 the free surface of the gland along its lateral border. No cystoid 

 follicles or deeply stained and degenerated nuclei are present. 



Embryo of 160 mm. (fig. 18). The only structures present 

 in the thyroid gland indicative of the presence of the ultimo- 

 branchial bodies are areas (C/), on each side lateral to the median 

 plane along the dorsal surface of the gland. In these areas the 

 follicles containing colloid are quite small in comparison to the 

 large majority present in the thyroid gland, but appreciably 

 larger than those found in corresponding areas in the thyroid of 

 145 and 150 mm. embryos. These areas extend from about the 

 caudal end of the anterior third well into the posterior fourth of 

 the thyroid gland which terminates in a rather blunt single proc- 

 ess. In the caudal end are a very few large follicles containing 

 colloid but it was impossible to determine whether or not they 

 developed in connection with the ultimob ranchial bodies. No 

 darkly stained or degenerated nuclei were found. 



Embryo of 175 mm. The follicles in the thyroid gland are on 

 an average considerably larger than those found in the preceding 

 stage. They vary greatly in size but are uniformly distributed 

 throughout the gland. No structures are present which can be 

 interpreted as derivatives of the ultimobranchial bodies. 



Embryo of 225 mm. (fig. 19). The only apparent traces of the 

 ultimobranchial bodies are areas of considerable extent in which 

 the follicles are comparatively small ([/). These areas are lo- 

 cated in the middle third on each side of the median plane and 



