THE HEART AND BLOOD-VESSELS. 167 



The fo iii' efferent branchial vessels of each side unite in the 

 ■dorsal wall of the phary nx to form the aorta : t he two aort ffi 

 are c ontinued forwards to the head as the carotid art eries, while 

 posteriorly they unite to form the single 'dorsal aorta, from 

 which branches arise supplying all parts of the body. 



The luuigs arise at a very early stage, but are for a long time 

 ■extremely small and of little functional importance. Each lung 

 receives blood from a branch of the fourth efferent branch ial 

 vessel (Jj'ig. 76, ap), an d returns it directly to the auricle by a 

 pilmn^f^i-ry Y'^^^" , '^^ As the tadpole increases in size, and the 

 lu ngs become of greate r importance, a s eptum appears, divi ding 

 the auricle into Hvstemio or venous, a nd pulmonary or arter ial 

 c avities. _S imultaneouslY with this, valves are formed in the 

 truncus arteriosus, by which the venous and arterial streams of 

 blood are kept apart to a certain extent. 



At the time of the metamorphosis the gill circulation is cut 

 off, by the establishment of direct communications between the 

 afferent and efferent branchial vessels, and the pulmonary cir- 

 culation becomes of much greater importance than before. 



2. The Heart . 



T he heart lies at firs t (Fig. 69, p. 146) o n the under surfa ce 

 of the head, belo w t.ha flnny of t he pharvnx. ab ove and slightly 

 ^"^inr* ^^° on^lfpi- and immediately in front of the commencing 

 liver. 



In this reg'ion the mesoblas t, as in the body generally 

 (Fig. 70, so, SP), is split into somatic a nd splanchnic layers, 



sepji.raterl liy a. distinct spaen Thig apar^p hapnTnpa thR ppri- 



cardial cavity ; the outer or somatic lay er of mesoblast formin g 

 the wall of th e pericardia l cavity ; and th e inner or splanchni c 

 la yer giving rise to the muscular wall r|f the h^nrt. 



The endothelial lining of th e heart is derived from a n umber 

 of scattered cells, which appear below the floor of the pharynx , 

 and" which are form ed partly, if not entirely, by direct pro- 

 liferation ot the hypoblast cells of the pharyngeal wall and of 

 the liver (cf. Fig. 69). These cells are at first irregularly ar- 

 ranged, but soon become disposed so as to form a tubular lining 

 to tho heart, which is for a time closed in front, while its 

 posterior wall is formed by the anterior surface of the liver 

 diverticulum (Figs. 64 and 69). 



