420 PHILOSOPHY OF ZOOLOGY. 
The most simple mode of hatching is effected by the 
situation in which the eggs are placed by the mother, after 

“« About the same time, the spine, which was originally extended in a 
straight line, becomes incurvated; and the distinction of the vertebra is 
very plain. The eyes may be distinguished by their black pigment, and 
comparatively immense size ; and they are afterwards remarkable in conse- 
quence of a peculiar slit in the lower part of the iris. 
‘¢ From the fourth day, when the chicken has attained the length of 
four lines, and its most important abdominal viscera, as the stomach, intes- 
tines, and liver, are visible, (the gall-bladder, however, does not appear til) 
the sixth day), a vascular membrane (chorion, or membrana wmbilicalis), be- 
gins to form about the navel, and encreases in the following days with such 
rapidity, that it covers nearly the whole inner surface of the shell within the 
membrana albuminis, during the latter half of incubation. This seems to sup- 
ply the place of the lungs, and to carry on the respiratory process instead of 
those organs. The lungs themselves begin, indeed, to be formed on the 
fifth day ; but, as in the fetus of the mammalia, they must be quite incapa- 
ble of performing their functions while the chick is contained in the amnion. 
“« Voluntary motion is first observed on the sixth day, when the chick 
is about seven lines in length. 
‘* Ossification commenees on the ninth day, when the ossific juice is first 
secreted, and hardened into bony points. These form the rudiments of the 
bony ring of the sclerotica, which resembles at that time a circular row cf 
the most delicate pearls. 
‘* At the same period, the marks of the elegant yellow vessels on the 
yolk-bag begin to be visible, 
‘* On the fourteenth day, the feathers appear; and the animal is now able 
to open its mouth for air, if taken out of the egg. 
*¢ On the nineteenth day, it is able to utter sounds; and on the twenty- 
first to break through its prison, and commence a second life. 
‘¢ The chorion, that most simple yet most perfect temporary substitute for 
the lungs, if examined in the latter half of incubation in an egg very cauti- 
ously opened, presents, without any artificial injection, one of the most 
splendid spectacles that occurs in the whole organic creation. It exhibits a 
surface covered with numberless ramifications of venous and arterial vessels. 
The latter are of the bright scarlet colour, as they are carrying oxygenated 
blood to the chick; the veins, on the contrary, are of the deep or livid 
ved, and bring the carbonated blood from the body of the animal. Their 
