200 CLASS AVES. 



transparence, and assume the forms and situations which 

 render them easy to be recognized. On this account they do 

 not become obvious to detection but at different periods. On 

 the first day, the head and dorsal spine may be distinguished ; 

 on the second, the vertebrae and the heart ; on the third, the 

 neck and breast ; on the fourth, the eyes and liver ; on the 

 fifth, the stomach and reins ; on the sixth, the lungs and 

 skin ; on the seventh, the intestines and bill ; on the eighth, 

 the gall-bladder and ventricles of the heart ; on the ninth, 

 the wings and thighs ; and on the tenth, all the parts which 

 should constitute the chicken are in their places, and have 

 already assumed the form which characterizes them. On the 

 subsequent days they are developed still more, acquire all 

 the growth of which they are susceptible in this state, and the 

 chicken becomes sufficiently strong to break the shell, which 

 takes place on the twenty-first day of incubation. 



The principle of life introduced by the act of the male 

 into the egg already commenced in the female ovary, pro- 

 bably contributes to organize it for the end which nature has 

 proposed. But as soon as this egg is laid, the principle of 

 life slumbers in it, until awakened by the caloric communi- 

 cated by the sitting hen ; then, in concert with this last 

 agent, it gives motion to the embryo which it is commissioned 

 to animate. It communicates to it the faculty of increase, of 

 employing in its organization and nutriment all the sub- 

 stances which are enclosed with it in this isolated matrix, 

 which, however, fulfils all the same functions as those of the 

 mammalia. 



The yolk of the egg then augments in quantity at the 

 expense of the albumen, the fluid part of which it absorbs. 

 It becomes a nutritious milk, which is carried into the liver, 

 elaborated there, and finally passes into the circulation. 



The yolk, to the nineteenth day of the incubation, forms in 

 the egg a distinct body from the bird, shut up in a separate 



