116 J. A. BADERTSCHER 



convinced that they do not represent a general degeneration ot 

 the iiltiniobranchial bodies. 



Follicles containing colloid appear first in the thyroid gland in 

 a 75 mm. embryo. In the ultimobranchial bodies the follicles 

 containing colloid are first quite numerous, though small (excep- 

 ting the cystoid follicles), in a 145 mm. embryo (fig. 19). A few 

 small follicles containing colloid were found in these structures 

 in the 125 mm. embryos. The retarded development of colloid 

 in the ultimobranchial bodies in the pig corresponds with a 

 similar retardation in its development in these structures in 

 the Echidna in which, according to Maurer ('99), they remain 

 independent structures. The time at which the transformation 

 of the ultimobranchial bodies into typical thyroid structures is 

 completed, that is, when they can no longer be distinguished 

 from the derivatives of the median thyroid anlage, is variable. 

 For example in a 175 mm. embryo their transformation is com- 

 plete while in No. 2 of the full term embryos the left one is com- 

 posed of an area of small follicles in which is located a small 

 area of cell cords free from colloid (fig. 21). A comparison of the 

 structure of the right ultimobranchial body, which is composed 

 of an area of small follicles, and the left one in No. 2 of the full 

 term embryos also shows that one ultimobranchial body may 

 undergo a more rapid transformation into typical thyroid struc- 

 tures than the other in the same embryo. 



Cell cords are formed from the periphery of the ultimobran- 

 chial bodies usually sooner than from their more central por- 

 tion, as stated above. It is also in the cell cords of the peripheral 

 portion of the ultimobranchial bodies that colloid is first formed, 

 so that the older peripheral follicles of these structures in many 

 stages are larger than the more centrally located ones. Figures 

 18, 19, and 21 show that the follicles containing colloid grad- 

 ually decrease in size toward the more central portions of these 

 structures. Since colloid appears first in the thyroid gland many 

 of the follicles are quite large before colloid is first formed in the 

 ultimobranchial bodies. It would thus seem that if the ingrowth 

 of structural elements of the thyroid gland into the ultimobran- 

 chial bodies is a factor in breaking up the latter into cell cords, 



