214 Mr. Home on the Mode of breeding 
there being two on each side of the egg, so that there is a 
current of sea water constantly supplying the egg with air. 
In the oviviviparous sharks the ova have no hard covering, 
being contained within the body of the fish, they are sur- 
rounded by the same jelly as in the frog, and the sea water 
is applied to the membrane in which they are inclosed, the 
opening of the oviduct admitting it for that purpose. 
In the bird, the eggs being surrounded by atmospherical 
air, are supplied with it through the porous texture of the 
shell, which readily transmits it to the membranes of the 
embryo. 
In all these tribes, the mode, in which the air is applied to 
the foetal blood is as follows. Besides the common circulation 
of the blood from the heart of the embryo, to the different 
parts of its frame, there is a lesser circulation from the arte- 
ries to the membranes, which inclose the embryo, and which 
are in contact with the aerated water, or air, in which the egg is 
deposited ; in this circulation, the blood attracts the air through 
the membrane, and conveys it into the system. In birds, the 
young are fed and taken care of by the parents, after the eggs 
are hatched, and in them this foetal circulation is immediately 
stopped, on the embryo breaking the shell, and the yolk is 
drawn up into the belly as a supply of nourishment, till the 
stomach has acquired the powers of digestion. 
In the oviviviparous sharks, the young ones swim about 
in the jelly, which surrounds the ova, with the yolk attached 
to the belly by a long vascular chord, till fitted to swim in 
the sea, and all this time the blood is aerated from these 
membranes. After the young leaves the bag in which the eggs 
are contained, the yolk is taken into the belly, as in the bird. 
