138 Elementary Zoology. 



air sacs, which are but prolongations of the lungs ; ' the 

 noiochord. 



The remaining organs of the body are derived from the 

 mesoblast. It will be necessary to go more into detail into the 

 description of the way in which some of these are developed. 



The Vascular System. — The heart arises as two closely 

 applied tubes, which soon fuse to become a single tube. 

 The walls of this tube are muscular without, and derived 

 from the mesoblast, and there is a lining of cells which 

 have been stated to arise from the hypoblast. It is a re- 

 markable fact that the heart begins to beat before the walls 

 are differentiated into muscular tissue.' Later on the heart 

 becomes twisted into an S shape, and constrictions appear, 

 marking it off into the several chambers of the adult heart. 



The anterior end of the heart gives off a series of aortic 

 arches, which, embracing the gut, unite upon its dorsal surface 

 to form a dorsal aorta, running back along the median dorsal 

 side of the gut. There are, altogether, five of these arches on 

 each side, just as in the tadpole. But, though the chick thus 

 resembles a tadpole or a fish, these aortic arches never send 

 off ramifications in those types, since no traces of actual gills 

 are developed. They simply lie between the gill clefts, and, 

 indeed, are related to the heart on the one hand (a tube, be 

 it remembered, at first) and to the dorsal aorta on the other, 

 as are the so-called hearts of the earth-worm to the ventral 

 blood-vessel and to the dorsal blood-vessel. The truncus 

 arteriosus, as the anterior end of the heart is termed, is 

 put into communication with the dorsally running aorta by 

 these five pairs of circular vessels. Later in development 

 the fish-like character of the aortic arches is lost. The 

 middle parts of the first two arches disappear ; the ventral 

 parts persist as an artery, supplying the tongue of the adult, 

 the lingual artery; the dorsal parts, as the carotids, which 

 run up the neck to the brain. The other arches are partially 

 or entirely lost, and the arterial system of the adult is arrived. 



The dorsal aorta gives off two great branches. The first 



' The muscular and connective tissue investments of these various 

 organs are derived from the splanchnic mesoblast, 



