4> C 20 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 vertebrae 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 till 

 the sixth day), a vascular membrane (cAorara, or membrana umbilicalis), 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. 

 44 Voluntary motion is first observed on the sixth day, when the chick 

 is about seven lines in length. 



44 Ossification commences 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 of 

 the most delicate pearls. 



44 At the same period, the marks of the elegant yellow vessels on the 

 yolk-bag begin to be visible. 



44 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. 



4 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. 



44 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 

 red, and bring the carbonated blood from the body of the animal. Their 



