VI 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 
