vin THE BIRD WITHIN THE EGG 279 



idea how the blood of a fish circulates. The heart is 

 a very imperfect one, and mixes the pure and impure 

 blood. It has one ventricle and one auricle. From 

 the former the blood is driven through a great artery 

 to the gills, and from them is gathered into another 

 great artery which branches and distributes it all over 

 the body. Thus, as a necessary consequence of the 

 absence of lungs, there is only one circulation. 

 The blood on leaving the heart is purified in the gills 

 and does not return to the heart before doing its work 

 in the body and limbs. Every one is familiar with 

 the gills of fishes. On the right and on the left sides 

 are arches of gristle which spring from the top of the 

 back part of the mouth and stand out, like bent bows, 

 on either side. Their frames of gristle or cartilage 

 are covered with delicate red fringes through which 

 the blood flows, separated only by a thin membrane 

 from the water which contains the oxygen it stands 

 in need of. In principle, therefore, they are the same 

 as lungs. The great artery, the aorta, which brings 

 the blood from the heart sends off branches to each 

 of the gills. Thus there are aortic arches as well as 

 gill arches (G, fig. 67). 



To return now to the chick. On the third day of 

 incubation, there are clear signs of arches such as I 

 have described. There are four clefts on each side, 

 and each cleft has a fold on its front border. The 

 fourth cleft has a fold both before and behind, and 

 thus there are five folds and four clefts. These folds 

 and clefts are homologous to the gill arches and clefts 

 in fishes. It is true they are now functionless. The 

 chick breathes throughout the twenty-one days of 



