. The structure and life of birds . are covered with delicate red fringes through whichthe blood flows, separated only by a thin membranefrom the water which contains the oxygen it standsin need of. In principle, therefore, they are the sameas lungs. The great artery, the aorta, which bringsthe blood from the heart sends off branches to eachof the gills. Thus there are aortic arches as well asgill arches (G, fig. 67). To return now to the chick. On the third day ofincubation, there are clear signs of arches such as Ihave described. There are four clefts on each side,and each cleft has a fold o
. The structure and life of birds . are covered with delicate red fringes through whichthe blood flows, separated only by a thin membranefrom the water which contains the oxygen it standsin need of. In principle, therefore, they are the sameas lungs. The great artery, the aorta, which bringsthe blood from the heart sends off branches to eachof the gills. Thus there are aortic arches as well asgill arches (G, fig. 67). To return now to the chick. On the third day ofincubation, there are clear signs of arches such as Ihave described. There are four clefts on each side,and each cleft has a fold on its front border. Thefourth cleft has a fold both before and behind, andthus there are five folds and four clefts. These foldsand clefts are homologous to the gill arches and cleftsin fishes. It is true they are now functionless. Thechick breathes throughout the twenty-one days of 28o THE STRUCTURE AND LIFE OF BIRDS chap. incubation, but not through its gill arches, nor throughthe lungs, which are not for some time connected with. Fig. 67.—(After Duval.) Embryo of sixty-eight hours, in, forebrain, to becomeoptic lobes ; B2, mid brain ; H3, hind brain, to divide into cerebellum and medullaoblongata ; ch, cerebral hemispheres ; E, eye ; ea, internal ear ; g, gill arches, withclefts between ; h, heart ; n, notochord, to pass into backbone ; ml, nasal pit. the heart, and do not set to work till respiration beginswith the hatching of the bird. The system of breath- VIII THE BIRD WITHIN THE EGG ing is peculiar to the embryo, and is carried on by meansof a wide extension of blood-vessels, rendered possibleby its protected situation within the shell. The roundspot, called the blastoderm, at the top of the egg, growsall round its circumference, and, its edges at lengthmeeting, it becomes a bag, with various pockets, whichenvelopes all the yolk. Out of the folds of the blasto-
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