228 THE CHICK. 



the heart and the liver. The limbs have greatly increased in 

 length ; their several segments are well established, and the 

 division of the distal ends into fingers and toes is very evident. 

 The white of the egg has almost disappeared, a thick and very 

 viscid mass, WA, alone remaining at the lower surface of the egg. 

 The yolk-sac, YS, is still large, but its walls are flabby, owing to 

 the absorption of a large part of its contents as food by the 

 embryo. The allantois, TA, has grown enormously, and has 

 spread over the back of the embryo, and quite half way round 

 the interior of the egg-shell. It lies close to the shell, so that 

 respiratory interchanges can readily take place, by diffusion 

 through the porous shell, between the gases of the blood, in the 

 vessels of the allantois, and the air outside the egg. In this way 

 the respiration of the embryo is effected. 



During the second half of the period of incubation the 

 changes are of less interest. The young chick steadily increases 

 in size at the expense of the yolk-sac, and gradually acquires 

 the proportions and characters which it has on hatching. About 

 the fourteenth day it shifts its position so as to lie lengthways 

 in the egg, rather than across it. On the twentieth or twenty- 

 first day the yolk-sac is nearly absorbed, and what remains of it 

 is drawn into the body of the chick, the body walls closing over 

 it at the umbilicus. The chick thrusts its beak through the 

 shell membrane into the air chamber at the broader end of the 

 egg, and for the first time draws air directly into its lungs. 

 Invigorated in this way, it breaks through the shell, by means of 

 a hard knob on the tip of its beak, and steps out into the world. 



THE EGG. 

 1 . Formation of the Egg-. 



In the embryo fowl there are two ovaries, but in the course 

 of development the right ovary disappears, and in the adult hen 

 the left ovary is alone present. This is a large, irregularly- 

 shaped body, suspended in a fold of peritoneum from the dorsal 

 body-wall, opposite the anterior part of the left kidney. Numerous 

 ova in different stages of development project from its surface, 

 varying in size from dust shot up to spherical bodies an inch or 

 more in diameter. 



Of the two oviducts, the right one is rudimentary ; the left 



