PLATE 4 



EXPLANATION OF FIGURES 



19 From a photograph of a portion of a section through the left ultimobran- 

 chial body and a portion of the thyroid gh\nd surrounding it. The ultimobran- 

 chial body is characterized by follicles which contain colloid and which are on 

 an average appreciably smaller than the follicles of the thyroid gland. From an 

 embryo 225 mm. long. X 38. 



20 From a photograph of a portion of a section through the right ultimobran- 

 chial body and a portion of the thyroid gland surrounding it. The ultimobran- 

 chial body contains many cystoid follicles which contain colloid. The colloid 

 dropped out from some of the follicles during the process of staining. From an 

 embryo 245 mm. long. X 38. 



21 From a photograph of a portion of a section through the left ultimobran- 

 chial body and a portion of the thyroid gland. The ultimobranchial body is 

 characterized by an area of small follicles in which is located a small area free from 

 colloid. The light dots represent follicles from which the colloid has fallen. 

 This figure represents the ultimobranchial body at C in figure 22 a. From No. 2 

 of the embryos 270 mm. long (full term). X 38. 



22 a, 22 b, and 22 c These figures show the relative size of the ultimobranchial 

 bodies and the thyroid gland in No. 2 of the embryos 270 mm. long (full term). 

 The extent of the ultimobranchial bodies is outlined by a dotted line. Inside 

 the left ultimoljranchial body is a small area (X), also outlined by a dotted line, 

 which is free from colloid (figs. 22 a and 22 c). The portion of the left ultimo- 

 branchial body outside the area X and all of the right one is characterized by 

 follicles which are on an average appreciably smaller than those of the thyroid 

 gland. Figures 22 b and 22 c represent cross sections through the tripartite 

 complex at b and c respectively of the structures represented in figure 22 a. X 

 7.5. 



T., thyroid U., ultimobranchial body 



130 



