104 J. A. BADERTSCHER 



caudalward these areas become larger, in places very irregular in 

 outline, in places broken up with typical thyroid structures, and 

 are located more deeply in the lateral halves of the thyroid gland. 

 The thyroid ends in a single blunt process that has a typically 

 thyroid structure. In a few places the ultimobranchial bodies 

 are unusually vacuolar. In these places the nuclei do not stain 

 deeply (U). Similar lightly stained areas were observed by 

 Kingsbury ('14) in the thyroid gland of human embryos. Also, 

 a few groups or nests of small, closely packed nuclei were found. 

 In some of these groups the nuclei had a normal structure, while 

 in others they were only slightly stained. A few degenerated 

 nuclei were found in the vacuolar areas and in their immediate 

 neighorhood. Deeply stained nuclei are present in small num- 

 bers in both vacuolar and nonvacuolar parts. 



Embryo of 60 mm. (fig. 13). The ultimobranchial bodies are 

 limited to the posterior third of the thyroid gland. The right one 

 is fused to the dorso-lateral margin of the gland and along its 

 greater extent is composed of loosely arranged cell cords (U). 

 Near the caudal end of the thyroid it merges into a vacuolar area 

 which is interspersed with typical thyroid structures. The nu- 

 clei of the cell cords are on an average smaller than those found 

 in the thyroid gland but, excepting a few darkly stained nuclei, 

 they have a normal structure. The left ultimobranchial body 

 lies just lateral to the median plane and is more deeply imbedded 

 in the thyroid than the right one. It is largely composed of 

 loosely arranged cell cords. Small vacuolar areas are present 

 throughout its entire extent. The tripartite complex ends in a 

 single process which is partly vacuolar. Mitoses can readily 

 be found in the loosely arranged cell cords. 



Embryos of 65 mm. The ultimobranchial bodies in two em- 

 bryos of this developmental stage are described in order to con- 

 trast the structure of these bodies in two embryos of the same 

 age. In one embryo these structures are represented by small 

 disconnected vacuolar areas which are promiscuously scattered 

 in the caudal half of the tripartite complex. In the other embryo 

 the ultimobranchial bodies are located in the posterior third of 

 the thyroid gland, and each one is characterized by a large and 



