FATE OF THE ULTIMOBRANCHIAL BODIES . 95 



cords and cell masses have a syncytial structure. The nuclei 

 vary in shape and somewhat in size but their form, average size, 

 and structure in this stage is the same as in the ultimobranchial 

 bodies. The nuclei of the thyroid are more closely packed to- 

 gether than in the vacuolar portions of the ultimobranchial 

 bodies, but when a nonvacuolar portion of the latter is brought 

 into the same microscopic field wdth a portion of the thyroid 

 gland, no difference in structure can be seen between them even 

 under high magnification (1500 diameters). Some of the spaces 

 between the cords of cells are lined with endothelium and con- 

 tain blood. 



Embryo of 19.5 mm. (figs. 2 a and 2 b). The ultimobranchial 

 bodies extend slightly farther cephalad than the thyroid gland. 

 Only slight traces of their lumen still persist. They lie along 

 the dorsal surface of the thyroid gland but are located nearer the 

 mesial plane than those of the preceding stage. In some places 

 there is actual fusion between these structures and the thyroid 

 gland (fig. 2 a, right side), while in other places a thin layer of 

 connective tissue intervenes (fig. 2 a, left side). The ultimo- 

 branchial body on the left side extends almost to the caudal 

 margin of the thyroid gland (fig. 2 b), w^hile on the right side it 

 terminates twelve sections (5 microns in thickness) earlier. The 

 shape and orientation of the thyroid gland is similar to that in 

 the preceding stage. 



In this stage, as in the preceding one, both vacuolar and non- 

 vacuolar areas are found in places along the periphery of the ul- 

 timobranchial bodies. In some places where actual fusion has 

 taken place wdth the thyroid gland it is impossible to tell w'here 

 the two striictures meet. Fusion with the thyroid gland has 

 taken place along the ventro-lateral surface of the ultimobran- 

 chial bodies. The dorso-medial surface of these structures is in 

 places studded with epithelial buds (fig. 2 a) . 



In the ultimobranchial bodies of this developmental stage are 

 found a few nuclei in which the nucleoplasm stains quite deeply 

 in comparison with that in the large majority of nuclei present. 

 In some of these nuclei the chromatin is more abundant than in 

 the more numerous and more lightly stained ones but in both 



