FATE OF THE ULTIMOBRANCHIAL BODIES 97 



Epithelial buds, as represented in figure 3 b (Ep.B.), are 

 present in various places along the free border of these structures. 

 These buds are fused to the more or less vacuolar mass of cells. 

 Mitosis can be found without much searching in both the ulti- 

 mobranchial bodies and the thyroid gland. 



Embryo of 21 nmi. (fig. 4). Both ultimobranchial bodies are 

 as long as the thyroid gland. They are fused to the latter along 

 their entire extent excepting the extreme caudal end of the left 

 one which is separated from the gland by a thin layer of con- 

 nective tissue. The one on the right side has a comparatively 

 regular outline and makes up nearly all of the lateral portion of 

 the tripartite complex ([/). The ultimobranchial body on the 

 left side is more deeply embedded in the thyroid gland than the 

 right one which makes it difficult to follow its extent in trans- 

 verse sections. In places blunt and both vacuolar and nonvacuo- 

 lar epithelial buds are attached to these structures. 



Groups of small nuclei in the ultimobranchial bodies are pres- 

 ent but they are not as mumerous as in the preceding stage. 

 The darkly stained nuclei are no more numerous than in the 

 previous stage. A few darkly stained nuclei were found in the 

 cell cords of the thyroid gland. These have a structure similar 

 to the darkly stained nuclei of the ultimobranchial bodies but 

 are not nearly as numerous and can be found only after prolonged 

 searching. Mitoses can readily be found in all the different 

 components of the tripartite complex. 



Embryo of 21.5 mm. (fig. 5). The ultimobranchial bodies lie 

 along the entire extent of the dorso-medial margin of the thyroid 

 gland and compose the largest portion of the tripartite complex. 

 The anterior extremity of both ultimobranchial bodies and the 

 posterior extremity of the left one are not fused with the thyroid 

 gland. Their largest diameter (U) is about midway between 

 their extremities from which they gradually taper to blunt 

 points. Their greater portion is vacuolar but nonvacuolar areas 

 are present in their deeper parts as well as along their periphery. 

 Large blunt epithelial buds, some of which are vacuolar, are 

 present in various places along their free border. The darkly 



