120 J. A. BADERTSCHER 



the latter grows more rapidly in size than the former so that in 

 stages from about 50 mm. in length to full term the ultimobran- 

 chial bodies usually lie in the posterior half of the thyroid gland 

 but may be found in the middle third or in the middle two-fourths 

 of the gland. 



3. The developmental stages in which the ultimobranchial 

 bodies transform into typical thyroid structures (that is, when 

 they can no longer be recognized structurally from the median 

 thyroid anlage) , vary greatly. The transformation of the greater 

 part of these structures may take place as early as in a 35 mm. 

 stage, before colloid is present in the thyroid gland, but in the 

 majority of stages examined it takes place in later stages. Even 

 in full term embryos an entire ultimobranchial body may not be 

 completely transformed. 



4. The ultimobranchial bodies in a thyroid gland may vary in 

 size, in shape, in the degree of their transformation, and in their 

 location in the lateral halves of the thyroid gland. This varia- 

 bility is particularly pronounced in some of the later develop- 

 mental stages. 



5. Colloid first appears in the follicles of the thyroid gland in 

 embryos of 75 mm. in length. A few small follicles containing 

 colloid appeared first in the ultimobranchial bodies of a 125 mm. 

 embryo. In a 145 mm. embryo the follicles containing colloid 

 in these structures are quite numerous although on an a\'erage 

 small in comparison with those in the thyroid gland. 



6. Large cystoid follicles containing colloid may develop in 

 the ultimobranchial bodies. 



7. The ultimobranchial bodies usually become entirely im- 

 bedded in the thyroid gland. In a few developmental stages 

 they were found to be only partially imbedded. 



8. The formation of cysts in the ultimobranchial bodies of pig 

 embryos is of rare occurrence. 



