110 J. A. BADERTSCHER 



along the dorsal surface of the thyroid gland, and compare in po- 

 sition to the ultimobranchial bodies in some other comparatively 

 late developmental stages. The areas of small follicles on the 

 right side is a little shorter than that on the left side. 



Embryo of 245 mm. (fig. 20). On the right side along the 

 lateral margin of the posterior two-thirds of the thyroid gland is 

 an area containing many cystoid follicles which contain colloid 

 and which are lined with cuboidal epithelium. This area, small 

 anteriorly, gradually becomes larger and reaches its greatest 

 cross-section area near the posterior fourth of the thyroid 

 gland. From this position it decreases in size and near its ter- 

 mination it is almost separated from the thyroid gland. This 

 area occupies a position similar to that of the ultimobranchial 

 bodies in some earlier stages and apparently represents a par- 

 tially imbedded ultimobranchial body similar to the right one 

 in No. 2 of the 125 mm. embryo (fig. 16a). 



On the left side lateral to the median plane and below the 

 dorsal surface of the thyroid gland is an area in which the aver- 

 age size of the follicles is appreciably smaller than the large ma- 

 jority of follicles in other portions of the thyroid gland. This 

 area lies in the posterior half of the thyroid gland but does not 

 extend as far caudally as the area of large follicles on the right 

 side. It also corresponds favorably in position to that most 

 generally occupied by the ultimobranchial bodies in earlier 

 stages. 



Embryos of 270 mm. (full term). The thyroid glands of two 

 full term embryos were examined. 



Embryo No. 1. The follicles containing colloid are variable 

 in size but uniformly distributed throughout the gland. The 

 only portion of the gland which can be interpreted as a deriva- 

 tive of an ultimobranchial body is an area of only small follicles 

 on the right side lateral to the median line in the posterior half 

 of the gland. This area extends through a series of only sixty 

 sections (10 microns in thickness) and lies hear the dorsal surface 

 of the gland. 



Embryo No. 2 (fig. 21). The thyroid gland extends through 

 a series of 827 sections (10 micrxjns in thickness). The left ulti- 



