184 



EMBRYOLOGICAL TYPES 



From the posterior end of the larynx, the lungs develop as 

 sacs stretching back parallel to the oesophagus on each side. 



-ti3. 



VC.3. 



Fig. 83. — Horizontal section through the head of a tadpole of Rana, showing 

 the formation of the visceral clefts (gill-slits). 



b 1, b 3, b 4, b 5, b 6, blood-vessels running in the first, third, fourth, fifth, 

 and sixth visceral arches (the vessels in the third and fourth arches will 

 become the carotid and systemic arches respectively) ; eg, external gills ; 

 1, infundibulum (floor of the forebrain) ; ic, internal carotid artery ; n, nasal 

 sac ; oe, oesophagus ; pd, pronephric duct ; pt, pronephric funnel ; sp, 

 splanchnoccel ; vc, 1 to 5, first to fifth visceral cleft (the first will give rise 

 to the Eustachian tube) ; V, VII, IX, branches of the trigeminal, facial, and 

 glossopharyngeal nerve, running in the first, second, and third visceral arches 

 respectively. 



The liver arises as a ventral outgrowth of the floor of the 

 gut, just in front of the mass of yolk-cells, and extending back 



