BIRDS. 73 



also possibly of use in changing the position of the centre 

 of gravity during flight. 



The heart has four chambers, the single ventricle of lower 

 forms being divided into right and left portions. The large 

 blood-vessels which lead from it are, in the embryo, much 

 like those of the fish ; but with development some parts are 

 altered and others suppressed, so that the result is more modi- 

 fied than in the forms already discussed. Thus the left half 

 of the third arch, except for an artery going to the wing of 

 that side, has entirely disappeared, while the right half, here 

 called the arch of the aorta, connects the left ventricle 

 with the dorsal aorta. From this the first arch, modified 

 into carotids, seems to arise. The second arch is completely 

 suppressed, while the fourth arch, arising from the right 

 ventricle, carries the blood to the lungs. In returning 

 from the body the venous blood is emptied into the right 

 auricle and passes thence, through the right ventricle, to 

 the lungs for aeration ; while that from the lungs goes to 

 the other side of the heart, and thence to all parts of the 

 body. Hence there is here no mixing of arterial and venous 

 blood in the heart. 



In the reproductive organs a constant feature is the 

 suppression of the right ovary, a rudiment of it existing in 

 a few forms. In the breeding-season the oviduct is very 

 large, and from its walls are secreted the white and the shell 

 of the egg. The eggs are large, and are always enclosed in 

 a limy shell. There is quite a difference in the condition in 

 which the young hatch from the egg. Some are nearly 

 naked and very helpless, while others are thickly clothed 

 with down and are able to run and to feed themselves. 



The brain is large, and, in comparison with the lower 

 forms already studied, is noticeable for the great develop- 

 ment of the cerebrum and cerebellum, which by their growth 



