THE LUNGS AND LIVER. 



155 



region of the mesenteron. This depression becomes still more 

 marked in the later stages (Fig. 69, w) ; and from its anterior 

 wall the liver is developed. 



Fig. 71. — Horizontal section of the head and body of a 12 mm. Tadpole ; 

 drawn from the dorsal surface, x 27. 



AP.l, 2, 3, 4, afferent branchial vessels of first, second, third, find fourth branchial 

 arches. BR. 3, cartilaginous bar of third branchial arch. EF.l, efferent branchial 

 vessel of first branchial arch. HB, basihyal cartilage. HY, cartilaginous bar of 

 hyoid ai-ch. J, jaw. KA, posterior end of archinephric duct, opening into cloaca. 

 LY, subcutaneous lymphatic spaces. OP, aperture leading from opercular chamber. 

 RA, right auricle of heart. RB, left auricle. BS, sinus venosus. RT, truncus 

 arteriosus. TD, duodenum. TI, intestine. VH, posterior vena cava. "W, liver. 

 "WD, bile duct. WG, gall bladder. Z, commencing hind limb. 



This anterior wall becomes early invested by mesoblast on 

 its outer surface, and in this mesoblast numerous blood-vessels 



