5i8 BIRDS. 



The allantois grows very rapidly, and forms, by the sixth or seventh 

 day, a large flattened sac, covering the right side of the embryo, and 

 continuing to spread between the folds of the amnion. 



The allantois becomes very vascular, and spreads over the yolk-sac ' ' as 

 a flattened bag filled with fluid ;" it secures the respiration of the embryo 

 by gaseous interchange with the surrounding medium. 



The white of egg diminishes very rapidly, especially after the sixth 

 day. After the tenth day, the yolk also begins to diminish rapidly, being 

 absorbed by the blood-vessels in the walls of the yolk-sac. 



By the eleventh day, the body of the embryo may be said to be 

 completed. It is still connected to the yolk-sac by a solid vitelline stalk, 

 which, along with the stalk of the allantois, forms the umbilical cord. 



On the third day, the blood is aerated in the capillaries of the yolk-sac; 

 on the fifth or sixth day, the allantois supplements the yolk-sac as a 

 respiratory organ ; afterwards, the whole of the respiration is secured by 

 the allantois ; finally, the chick begins to use its lungs. With these 

 changes, important alterations in the vascular system are necessarily 

 connected. 



As early as the sixth day, movements are exhibited by the limbs of 

 the embryo. " They cannot be of any great extent until the fourteenth 

 day, for up to this time, the embryo retains the position in which it was 

 first formed, viz., with its body at right angles to the long axis of the 

 egg. On the fourteenth day, a definite change of position takes place ; 

 the chick moves so as to lie lengthways in the egg, with its beak touching 

 the chorion (or outer fold of the amnion) and the shell-membrane, where 

 they form the inner wall of the rapidly increasing air-chamber. " 



"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 circulation becomes functionally 

 active, and at the same time blood ceases to flow through the umbiUcal 

 arteries. The allantois shrivels up, the umbilicus becomes completely 

 closed, and the 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." 



Classification of Birds. 



Order i. Saururie. Ancient extinct birds, with reptilian affinities more 

 marked than in any living forms. 



The oldest known bird is Archaopteryx, remains of which have been 

 found in the Solenhofen slates in the Upper Oolite (Jurassic) of Bavaria. 

 " The stone is so fine grained that, besides the bones of the wings, the 

 furculum or merrythought, the pelvis, the legs, and the tail, we have 

 actually casts or impressions on the stone (made when it was as yet only 

 soft mud) of all the feathers of the wings, and of the tail. " 



This link between Birds and Reptiles seeems to have been a land bird 

 about the size of a crow. It had feathers and also teeth. In the fossil 

 specimen the feathers are confined to the wings, legs, and tail, those on 

 the head, neck, and trunk, having perhaps fallen off. Each joint of the 

 long tail bears a. pair of lateral feathers — a unique arrangement. The 



