VIII 



THE BIRD WITHIN THE EGG 



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- 



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 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, 



