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 
