476 The Bird 



have been observed, instantly bringing to mind that some- 

 what gull-like, toothed bird of old Icthyornis. 



The origin and subsequent changes, in the embryo chick, 

 of the vascular system, including the heart, nerves, and 

 arteries, are more intricate than the development of any 

 other system of organs, and for an excellent reason. We 

 know that the frog's egg hatches as a tadpole, which breathes 

 by means of gills and lives, for a considerable time, in the 

 water. We learned in Chapter IV that important parts 

 of the head and sense-organs of birds are derived from 

 metamorphosed gills ; so the inference is that all the changes 

 in the blood-channels, which in the tadpole and frog take 

 place during several months, are in the embryo chick 

 gone through with in a period of a few days. 



The blood in the heart of a fish is sent from the single 

 ventricle to the gills, and from there it is distributed all 

 over the body. In the gills it passes through the paired 

 series of red fringes and is oxygenated by the water. Now 

 in the chick there are six pairs of these gills, or paired 

 blood-vessels (although not more than three or four are 

 found at one time). The chick breathes by means of a 

 membranous sheet of blood-vessels spread out just beneath 

 the shell, and even the lungs are not brought into use until 

 just before the bird hatches. But strange to say, although 

 there is no water to supply the gill-channels with life- 

 giving oxygen, yet blood actually flows through them, in 

 obedience to the long-forgotten ancestral life-habits 

 useless these many millions of years. 



Of all the gill-channels, but three remain in the adult 

 bird. The great aorta, which springs from the heart and 



