nes. In the
mammal they are biconcave disks with bevelled edges, they do not contain
a nucleus so that they are not cells. In the bird they are larger,
ellipsoidal in shape and have a large nucleus in the centre of the cell.
In reptiles and amphibia the red corpuscles are also nucleated, but the
_stroma_ portion containing the haemoglobin is arranged in a thickened
annular part encircling the nucleus. When seen from the flat they are
oval in section. In fishes the corpuscles show very much the same
structure. A further very significant difference to be observed between
the bloods of different vertebrates is in the amount of haemoglobin they
contain; thus in the lower classes, fishes and amphibia, not only is the
number of red corpuscles small but the amount of haemoglobin each
corpuscle contains is relatively low. The concentration of the
haemoglobin in the corpuscles attains its maximum in the mammal and the
bird. Since the haemoglobin is practically the same from whatever animal
it is obtained and can only combine with the same amount of oxygen, the
oxygen-capacity of the blood of any vertebrate is in direct proportion
to the amount of haemoglobin it contains. Therefore we see that as we
ascend the scale in the vertebrate series the oxygen-carrying capacity
of the blood rises. This increase was a natural preliminary condition
for the progress of evolution. In order that a more active animal might
be developed the main essential was that the chemical processes of the
cell should be carried out more rapidly, and as these processes are
fundamentally oxidative, increased activity entails an increased rate
of supply of oxygen. This latter has been brought about in the animal
kingdom in two ways, first by an increase in the concentration of the
haemoglobin of the blood effected by an increase both in the number of
corpuscles and in the amount of haemoglobin contained in each, and
secondly by an increase in the rate at which the blood has been made to
pass through the tissues. In the lower vertebrates the blood pressure is
low and the haemoglobin content of the blood is low, consequently both
rate of blood-flow and oxygen-content are low. In contrast with this, in
higher vertebrates the blood pressure is high and the haemoglobin
content of the blood is high, consequently both rate of blood-flow and
oxygen-content are high. We must associate with this important step in
evolution the means employed for the more rapid absorption