. The structure and life of birds . 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, re
. The structure and life of birds . 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-. Fig. 68.—(«) after Gadow. Transverse section through embryo during third day ;AL, allantois : later it spreads out and lies, full of blood-vessels, close under the shell ;am, amnion, a fold formed from embryo and enveloping it ; M, spinal marrow ; N,notochord, to pass into backbone ; ys, yolk-sack. (6) Diagram of the circulation ofthe yolk-sack at the end of the third day of incubation. The veins are marked inoutline, and the arteries in black, h, heart (after Foster and Balfour). derm the chick is formed ; it lies like a crease in thewalls of the bag, and sends out its blood-vessels intotwo of the pockets, one called the yolk-sack, theother the allantois, which forms a branch of thealimentary canal. The first few days the work isdone mainly in the yolk-sack ; later on the allantoisplays the more prominent part. The shell, of course, 282 THE STRUCTURE AND LIFE OF BIRDS chap. is porous, and allows of the passage of gases outand in. All the pairs of arches except three, the first, se
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