108 J. A. BADERTSCHER 



lined with cuboidal and columnar epithelium. Some of the 

 nuclei in the syncytial masses and in the coarse cell cords stain 

 more deeply than others, and in general they are more variable 

 in size than those in the follicular epithelium of the thyroid gland. 

 The nuclei in the epithelial lining of the cystoid folicles lie closely 

 together and stain uniformly. While the cystoid follicles are 

 free from colloid in this developmental stage, small follicles con- 

 taining colloid are thinly scattered throughout its free portion 

 (fig. 16 a, Co). Along the line of fusion of the free portion of 

 the ultimobranchial body to the thyroid gland there is in the 

 latter an area composed of cell cords in which colloid is just be- 

 ginning to form (fig. 16 a). The cell cords of this area have a 

 structure similar to those in earlier stages in which colloid forma- 

 tion has just begun. In the free portion of this structure vacu- 

 oles are almost entirely lacking and only a few degenerated 

 nuclei were found. 



The length of the ultimobranchial body on the left side is 

 equal to the length of the right one. It also lies along the lateral 

 margin of the thyroid gland but is more deepl}^ imbedded in it 

 (fig. 16 b, U). In cross section it is smaller than the right one 

 but, excepting the absence of cystoid follicles, it has a similar 

 structure. By referring to the figure it will be seen that it 

 merges gradually into the thyroid gland. The follicles contain- 

 ing colloid gradually become smaller toward the more central 

 portion of the ultimobranchial body in which only an occa- 

 sional small follicle can be found. 



Embryo of 145 mm. (fig. 17). The ultimobranchial bodies on 

 both sides are limited to the middle two-fourths of the thyroid 

 gland just lateral to its median plane. The right one along 

 nearly its entire extent is partly exposed to the free surface along 

 the dorsal border of the gland. The portion most deeply im- 

 bedded in the thyroid gland is represented by an area of cell 

 cords in which the follicles containing colloid are quite numerous 

 but all very small (U). _ In places along its free margin are 

 found cystoid follicles which also contain colloid {C.F.).^ 



' The substance in the cystoid follicles is called 'colloid' on the ground that it 

 has a staining reaction identical to that of the colloid in the follicles of the thy- 

 roid gland. 



