GILL-CLEFTS 



555 



With the anterior part of the ahmentary canal two strange 

 structures are associated — the thyroid and the thymus. 



Fig. 314. — Diagram of embryonic pharynx in a MammaL- 



After Goette. 



D., Dorsal surface ; PH., cavity of pharynx ; I-IV, gill-clefts 

 growing out ; TH., thyroid downgrowth ; C, ccelom ; 

 N., indications of nephridia. The deep black inner line is 

 endoderm ; the cross-hatched outer line is ectoderm ; the 

 dotted portion is mesodermic tissue. 



The thyroid gland arises as a diverticulum from the ventral wall of 

 the pharynx. It may be single (as in some Mammals), or bilobed (as 



