FATE OF THE ULTIMOBRANCHIAL BODIES 113 



small vacuolar syncytial masses entirely surrounded by typical 

 thyroid structures (fig. 12). 



Up to about a 24 mm. stage a marked contrast exists in the 

 structure of the ultimobranchial body and the thyroid gland, in 

 that the former are largely unbroken syncytial masses, \\'hile the 

 latter is broken up into cell cords (as seen in cross section) . Al- 

 though epithelial buds produce irregularities on the surface of 

 the ultimobranchial bodies even in a 21 mm. stage and indica- 

 tions of cell cord formation were found in one 22 mm. embryo, 

 the process of extensive cell-cord formation in these structures 

 is particularly active in stages ranging from 24 to 27 mm. in 

 length. The larger caudal end becomes broken up somewhat 

 later than the smaller anterior end. Usually, also, the more cen- 

 tral portion breaks up into cell cords later than the periphery. 

 The syncytial cords when first formed are usually larger or 

 coarser than those of the thyroid gland. Many are vacuolar for 

 some distance away from the central more or less vacuolar core 

 to which they may be attached. The time of breaking up of the 

 central core into cell cords is very variable. The extent to 

 which the ultimobranchial bodies become invaded with vascular 

 tissue corresponds closely to the extent of cell cord formation. 

 The first blood vessels, which are of a capillary nature, are found 

 in these structures in a 23 mm. embryo. 



According to Simon ('96) the cell cores of the ultimobranchial 

 bodies are formed in an entirely passive way, namely, by the 

 ingrowth of vascular tissue and of structural elements of the 

 median thyroid. That the former is a potent factor in this proc- 

 ess is, I believe, beyond doubt. It appears to me, however, 

 that he lays too much stress on the formation of cell cords by the 

 ingrowth of thyroid structures which will be considered later. 

 Another active factor in the process of cell cord formation is a 

 continued growth and branching of the epithelial buds found on 

 their surface in early stages. The buds by continued growth and 

 branching take the form of coarse cell-cords which can in many 

 instances be recognized structurally from the smaller cell cords 

 of the thyroid gland by the larger proportion of small nuclei 

 which they contain and by vacuoles which, when present, are 



