554 ZOOLOGY. 
zard are indicated, and the beak begins to develop. By the 
ninth or tenth day the feathers originate in sacs in the 
skin, these sacs by the eleventh day appearing to the naked 
eye as feathers; the claws and scales of the legs and toes are 
marked out on the thirteenth day, and by this time the 
cartilaginous skeleton is completed, though the deposition 
of lime (ossification) begins on the eighth or ninth day by 
small deposits of bone in the shoulder-blade and limb-bones ; 
centres of ossification appearing in the head by the thir- 
teenth day. 
“* After the sixth day, muscular movements of the embryo 
probably begin, but they are slight until the fourteenth day, 
when the embryo chick changes its position, lying length- 
ways in the egg, with its beak touching the chorion and 
shell membrane, where they form the inner wall of the 
rapidly increasing air-chamber at the broad end. On the 
twentieth day or thereabouts, the beak is thrust through 
these membranes, and the bird begins to breathe the air 
contained in the chamber. Thereupon the pulmonary cir- 
culation becomes functionally active, and at the same time 
blood ceases to flow through the umbilical arteries. The 
allantois shrivels up, the umbilicus becomes completely 
closed, and thé chick, piercing the shell at the broad end 
of the egg with repeated blows of its beak, casts off the 
dried remains of allantois, amnion, and chorion, and steps 
out into the world.’? (Foster and Balfour.) 
Some young birds have, as in turtles and snakes, a tem- 
porary horny knob on the upper jaw, used to crack the 
shell before hatching. In birds which lay small eggs, with 
a comparatively small yolk, the young are brooded in nests 
and fed by the parent ; but in the hen and other gallina- 
ceous birds, in the wading birds and many swimmers, as 
ducks, where the yolk is more abundant, the young main- 
tain themselves directly on hatching. 
Following the business of reproduction is the process of 
moulting the old and weather-beaten feathers. This is often 
a critical period in a bird’s life, judging by the occasional 
mortality among domesticated and pet birds. The annual 
moulting begins at the close of the breeding season, though 
