112 J. A. BADERTSCHEU 



cipally by cystic formations (guinea-pig, rabbit, cat, calf, sheep) 

 or their complete disappearance (pig) . 



The ultimobranchial bodies first fuse with the thyroid gland 

 along their ventro-lateral border. In early stages (19 mm. to 

 about 27 mm.) they make up a considerable portion of the horns 

 of the crescent-shaped tripartite complex. The extent of their 

 fusion to the thyroid gland during their period of retarded 

 growth (from about 33 mm. to full term) is variable. In the ma- 

 jority of late stages they are entirely imbedded in the thyroid 

 gland while in some they are only partially imbedded. The 

 latter condition is particularly the case in the following embryos; 

 48 mm. (figs. 11 a and 11 b); 60 mm. (fig. 13) ; 100 mm. (fig. 14); 

 125 mm. (fig, 16 a and 16 b); and 145 mm. (fig. 17). In the 

 later stages they usually lie more or less deeply imbedded below 

 the dorsal surface of the thyroid gland lateral to its medial plane, 

 but occur less frequently along the lateral or dorso-lateral margin 

 of the gland. 



The formation of vacuoles in the ultimobranchial bodies be- 

 gins before their fusion with the thyroid gland has occurred and 

 continues after fusion. However, in the various stages examined 

 no ultimobranchial body was found that is vacuolar throughout. 

 In human embryos Kingsbury ('14) finds that vacuolation, 'retic- 

 ulation,' continues until the entire structure is altered in this 

 way. The extent to which vacuolation takes place varies in 

 embryos of the same and different developmental stages. For 

 example in No. 1 of the 125 mm. embryos the more central 

 portion of these structures are quite vacuolar while in No. 2 of 

 the 125 mm. embryos no vacuoles are present. Also no vacuoles 

 are present in the left ultimobranchial body in No. 2 of the 27G 

 mm. embryos. In early stages non-vacuolar portions are present 

 along the periphery as well as in the deeper portions of these 

 structures. In later stages in which the ultimobranchial bodies 

 are largely broken up into cell cords the vacuoles are most numer- 

 ous in their more central unbroken portion although vacuolated 

 syncytial cords were found. In a few stages of which the em- 

 bryo 53 mm. long is an example, the only part of the ultimo- 

 branchial body that can be recognized structurally as such are 



