VIII THE BIRD WITHIN THE EGG 281 
ing is peculiar to the embryo, and is carried on by means 
of a wide extension of blood-vessels, rendered possible 
by its protected situation within the shell. The round 
spot, called the blastoderm, at the top of the egg, grows 
all round its circumference, and, its edges at length 
meeting, it becomes a bag, with various pockets, which 
envelopes all the yolk. Out of the folds of the blasto- 
WS 
E oD; PY’ Pp 
SR 
Fic. 68.—(a) 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; mM, spinal marrow ; N, 
notochord, to pass into backbone; ys, yolk-sack. (6) Diagram of the circulation of 
the yolk-sack at the end of the third day of incubation. The veins are marked in 
outline, and the arteries in black. H, heart (after Foster and Balfour). 
derm the chick is formed ; it lies like a crease in the 
walls of the bag, and sends out its blood-vessels into 
two of the pockets, one called the yolk-sack, the 
other the allantois, which forms a branch of the 
alimentary canal. The first few days the work is 
done mainly in the yolk-sack ; later on the allantois 
plays the more prominent part. The shell, of course, 
