THE HYPOMERE (LATERAL PLATE MESODERM) 



239 



inally the anterior extensions of the dorsal aortae. They proceed to 

 the base of the skull and give rise to the palatine (to the roof of the 

 mouth), the cerebral (to the brain), and the ophthalmic (to the eyes) 

 arteries. The original paired ventral aortae proceed into the head as 

 the smaller external carotids and join the blood islands and sinuses 

 of the floor of the mouth and the tongue as the lingual arteries. The 

 carotid glands, derived from the third visceral arch and having the 



Never develop 



Internal carotid 



External carotid 



Verteoral artery 



I -/— Dorsal aorta 

 Fate of the aortic arches of the frog embryo. 



function of filtering carotid blood, are located at the bifurcation of 

 the internal and the external carotids. This is at the original ventral 

 origin of the first branchial arch. 



The aortic arches which pass through the second branchial (fourth 

 visceral) arch become greatly enlarged and retain their connection 

 with the dorsal aortae on both sides, to become the main systemic 

 trunk. That portion of the dorsal aorta anterior to the second bran- 

 chial arch then degenerates, so that all of the arterial blood of the 

 first branchial arch must pass anteriorly and all of the blood of the 

 second branchial arch must pass posteriorly. The systemic trunk on 



