CHAPTER III. 



DEVELOPMENT OE THE PHAKYNX AND NECK. 



Pharynx of the Embryo. — There is very little resemblance 

 between the pharynx and neck of a human foetus in the third 

 week and that of the adult (Figs. 15^4 and B, p. 18). Indeed, 

 at the third week the human pharynx resembles closely that 

 of a fish (Figs. 2 1 A and B). In both the human foetus and 

 fish the pharynx is bounded by visceral or branchial arches, which 

 are separated by depressions (human embryoes) or clefts (fishes) ; 



oto-cyst 



sinus uenosus 



cerebral uesicle 



olfact pit 

 max. proc. 



■stomodaeum 

 of 1st (mandib) arch 

 artery of 2nd arch 

 irtery of 3rd arch 



bulbus arteriosus 

 dorsal aorta 



ventricle 



Fig. 21.4.— Showing the Visceral Arches and Cleft-depressions in the Pharyngeal 

 Wall of a 4th week human Embryo. Bach Visceral Arch contains an Aortic Arch 

 (After His.) 



in both the heart is situated under the pharynx, and from the ventral 

 aorta aortic arches pass up on each side, one in each visceral arch, 

 to terminate in the dorsal aortae. In fishes the aortic arches give 



