102 J. A. BADERTSCHER 



caudal one-fifth of the tripartite complex. The greater portion 

 of the ultimobranchial bodies are in the form of vacuolar syncy- 

 tial cores which give off coarse cell cords. Some of the coarse 

 cell cords are vacuolar and many are fused to the cell cords of 

 the thyroid gland. The central core is more or less' invaded 

 with mesenchymal and vascular connective tissue. Deeply 

 stained nuclei are quite numerous in the. ultimobranchial bodies 

 and a few are found in the cell cords of the thyroid gland. No 

 degenerated nuclei were found. The variableness in the size of 

 the nuclei in the ultimobranchial bodies is more marked than in 

 the nuclei of the thyroid gland, the former having a proportion- 

 ally larger number of small nuclei. The extent to which the 

 transformation of the ultimobranchial bodies has taken place in 

 this stage is about equal to that in the 29.5 mm. embryo. 



Embryo of 38 mm. The ultimobranchial bodies are limited 

 to the posterior two-thirds of the tripartite complex. Their an- 

 terior ends are small and entirely imbedded in the thyroid gland 

 near its dorsal surface. Caudalward they rapidly increase in 

 size. The tripartite complex ends in two blunt cone-shaped proc- 

 esses the greater portion of which are composed of the ultimo- 

 branchial bodies. The ultimobranchial bodies are composed of 

 irregularly outlined syncytial cores which gradually merge into 

 the compactly arranged cell cords, of the thyroid gland. Only a 

 few capillaries are found in them. Mitoses in the vacuolar areas 

 as well as in the cell cords of the thyroid are quite numerous. 

 Only a few deeply stained nuclei are present. No degenerated 

 nuclei were found. 



Embryo of 40 mm. The ultimobranchial bodies lie in the pos- 

 terior half of the thyroid gland. Their anterior parts are repre- 

 sented by small disconnected vacuolar areas which lie deeply 

 buried in the thyroid gland lateral to its median plane. Caudal- 

 ward these areas become large and branched so that the caudal 

 fifth of the thyroid gland is largely invaded by a vacuolar syncy- 

 tial mass which is not sharply demarcated from the closely 

 packed cell cords of the thyroid gland. The tripartite complex 

 ends in two blunt and slightly vacuolar processes of unequal 

 length. A small number of deeply stained nuclei are present 



