PLATE 3 



EXPLANATION OF FIGURES 



12 From a photograph of a small portion of a section through an ultimobran- 

 chial ])ody (the light area in the figure) which merges gradually into typical thy- 

 roid structures. In the light area the majority of the ultimobranchial nuclei do 

 not stain deeply. From an embryo 53 mm. long. X 337. 



13 From a photograph of a portion of a transverse section of the tripartite 

 complex showing the right ultimobranchial body which is largely composed of 

 coarse and loosely arranged cell cords. From an embryo 60 mm. long. X 60. 



14 From a photograph of a portion of a section of the tripartite complex show- 

 ing the right ultimobranchial body which, in this particular place, is composed of 

 loosely arranged cell cords in which no colloid is present. From an embryo 100 

 mm. long. X 56. 



15 From a photograph of a portion of a transverse section through the pos- 

 terior portion of the tripartite complex showing the compactly arranged cell 

 cords of the right ultimobranchial body in which is located a cyst. The area in- 

 side the dotted circle is free from colloid. From an embryo 125 mm. long. X 45. 



16 a and 16 b From photographs of portions of a transverse section of ths 

 tripartite complex showing respectively the right and left ultimobranchial bodies. 

 The right one is only partially imbedded in the thyroid gland and contains many 

 cystoid follicles (C.i^.)'which do not contain colloid and a few small follicles 

 which contain colloid (Co). The black dots in the portion of the figure labeled 

 'thyroid' represent colloid. The left ultimobranchial body is more deeply im- 

 bedded in the thj-roid gland. From an embryo 125 mm. long. X 38. 



17 From a photograph of a portion of a transverse section of the trip:.rtite 

 complex showing the right ultimobranchial body in which are found both small 

 and cystoid follicles that contain colloid. From an embryo 145 mm. long. X 38. 



18 From a photograph of a portion of a section of the tripartite complex 

 showing the left ultimobranchial body which is represented by an area of small 

 follicles. The black dots in the figure represent colloid. From an embryo 160 

 nun. long. X 38. 



C, cyst T., thyroid gland 



C.T., cystoid follicles U., ultimobranchial body 



Co., colloid 



128 



