THE COUESE OF THE CIRCULATION. 315 



(iv) Closure of the foramen ovale. This is not completed 

 until some little time after hatching. Its eflfect is to absolutely 

 prevent the passage of blood from the right to the left auricle: 

 Prom the time of its completion, the blood brought to the heart 

 by all three venae cavse is discharged into the right auricle, 

 and passes from this into the right ventricle ; while the only 

 blood entering the left auricle is that brought to it from the 

 lungs by the pulmonary veins. 



On the completion of these changes the circulation becomes 

 that of the adult bird. The arterial and venous streams of blood 

 are kept quite distinct, and the so-called double circulation is 

 completely established. 



DEVELOPMENT OF THE URINARY ORGANS. 



1. General Account. 



The urinary organs of the chick, while agreeing in their 

 general relations and mode of development with those of the 

 frog, yet present considerable and important points of differ- 

 ence. 



The head kidneys, which in the young tadpole are of large 

 size, and for a considerable time are the sole excretory organs 

 present, are in the chick extremely rudimentary structures, 

 which appear later than the Wolffian bodies, and disappear 

 again almost at once. 



The Wolffian bodies of the chick are developed early : they 

 soon attain a large size, and form the functional excretory organs 

 during embryonic life. Shortly after the time of hatching, they 

 lose their kidney structure and excretory function completely, 

 though parts of them persist as accessory portions of the repro- 

 ductive apparatus. 



The Wolffian and Miillerian ducts develop independently in 

 the chick, at any rate so far as their anterior ends are concerned. 

 As in the frog, the Miillerian duct becomes the ovidact of the 

 female, while the vas deferens of the male is formed from the 

 Wolffian duct. 



2. The Wolffian Duct. 



The Wolffian duct, which is the first part of the kidney 

 system to be developed, appears early in the second day, in 



