106 J. A. BADERTSCHER 



present. A few deeply stained nuclei are present. Some are 

 also found in the cell cords of the thyroid gland. A few degen- 

 erated nuclei were found. The ultimobranchial body on the 

 left side has a similar structure to the one on the right side but 

 lies deeply buried below the dorsal surface of the gland. The 

 portion of the tripartite complex which can be distinctly recog- 

 nized as a derivative of the ultimobranchial bodies, and the cell 

 cords in their immediate neighborhood contain no colloid al- 

 though the cell cords have a typical thyroid structure of some- 

 what earlier stages. 



Embryo No. 2. The ultimobranchial bodies cannot be iden- 

 tified structurally. However, it is to be noted that on the right 

 side, lateral to the median plane and along the dorsal border in 

 the middle third of the gland is an area which contains no col- 

 loid. This area is composed of closely packed cell cords which 

 have the same structure as the cell cords of the thyroid just 

 before the appearance of colloid, such as in a 60 mm. embryo. 

 On the left is an area similar in structure to the one on the right 

 side only its cephalo-caudal extent is considerably less. These 

 areas which are free from colloid correspond favorably in position 

 to that of the ultimobranchial bodies in some of the previous 

 stages. The thyroid terminates in two rather blunt processes 

 the extreme caudal portions of which contain no colloid. 



Embryo of 111 mm. The ultimobranchial body on the right 

 side of the gland is represented by two small and widely sepa- 

 rated syncytial masses which extend through a series of ten and 

 six sections respectively (10 microns in thickness), and on the 

 left side it is represented by a syncytial mass extending through 

 a series of eleven sections. These syncytial masses are quite 

 vacuolar and the nuclei are comparatively small and clear and 

 many are irregular in outline. Only a few darkly stained and 

 degenerated nuclei are present. 



It is of importance to note that on the right side lateral to the 

 median plane in the middle two-fourths of the gland and in line 

 with the syncytial masses described above is an irregularly out- 

 lined area of closely packed cell cords. This area is quite large 

 in cross section and is free from colloid. A similar area of about 

 the same length is present in the left side of the thyroid but it 



