634 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 nails 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 ai-e 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 the chick, piercing the shell at the broad end 

 of the Qgg with repeated blows of its beak, casts off the 

 dried remains of allantois, amnion, and chorion, and steps 

 out into the world. " (Poster 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 



