320 J. A. BADERTSCHER 



distinguished from each other without difficulty. The sinus 

 cervicahs, formed by the rapid growth in a caudal direction of 

 the mandibular and hyoid arches and the more retarded growth 

 of the branchial arches proper, is already well mapped out. As 

 this stage shows well accepted relations and developmental steps 

 it needs no further description. 



Embryo of lJf-.5 mm. {figures 1 and 2). This is the youngest 

 developmental stage from which a reconstruction of one side 

 of the neck was made. Since the determination of the real 

 origin of the thymus was one of the crucial points in this inves- 

 tigation only that part of the neck containing the anterior por- 

 tion of the thymus. anlage and the vesicula cervicalis was modelled. 



The posterior edge of the hyoid arch has grown over the open- 

 ing of the sinus cervicalis, shutting it off from the exterior. The 

 cavity thus formed is the vesicula cervicalis (F.c.) or the 'vesicula 

 thymica' of Kastschenko.^ The vesicula cervicalis, now widely 

 separated from the ectoderm (Ect.), is still connected with it 

 by a heavy cord of cells, the ductus cervicalis (D.c). Only in 

 places through its entire extent are traces of a lumen left. To 

 the outer end of the vesicula cervicalis is attached a cord of cells 

 that runs in an antero-ventral and mesial direction and con- 

 nects with the second pharyngeal pouch. This is the ductus 

 branchialis (D.b.). The anterior one-fifth of this structure pos- 

 sesses a lumen which is continuous with that of the second pouch. 

 At this stage it is impossible to determine the extent of the part 

 that is of entodermal origin and the extent that is of ectodermal 

 origin. The boundary line between the two has disappeared 

 through the obliteration of the lumen. Fox ('08) was unable 

 to find the ductus branchiahs in pig embryos but demonstrated 

 a long diverticulum — 'filiform process' — arising from the ventro- 

 lateral angle of the second pouch and connected with the ecto- 



1 "According to H. Ilabl ('09) the term 'vesicula cei'vicalis' is to be applied 

 to the entire complex, including the two ductus branchiales; Hammar uses the 

 term 'vesicula praecervicalis' only for the vesicular portion that is associated 

 with the third pharyngeal pouch, this portion being approximately identical 

 with the 'fundus praecervicalis' (cervicalis) of His and H. Rabl, as well as with 

 the 'vesicula thymica' of Kastschenko and the 'sinus vesicle' of Zuckerhandl." 

 (Quoted from Keibel and Mall's Human embryology, vol. 2, p. 456). 



