ce in starting the process of
clotting.

They are constantly suffering destruction in the performance of their
work. Many, too, are lost to the body by their passage through the
different mucous surfaces. Their origin is still obscure in many points.
The lymphocytes are derived from lymphoid tissue, wherever it exists in
the different parts of the body. The polymorphonuclear and eosinophile
cells are derived from the bone-marrow, each by division of specific
mother cells located in that tissue. The macrocyte is believed by many
to represent a further stage in the development of the lymphocyte. Their
rate of formation may be influenced by a variety of conditions--for
instance, they are found to vary in number according to the diet and
also, to a considerable extent, in disease.

_Platelets._--The platelets or thrombocytes (Gr. [Greek: thrombos],
clot) are the third class of formed elements occurring in mammalian
blood. There are still, however, many observers who consider that
platelets are not present in the normal circulating blood, but only make
their appearance after it has been shed or otherwise injured. They are
minute lens-shaped structures, and may amount to as many as 800,000 per
cubic mm. Under certain conditions, examination has shown that they are
protoplasmic and amoeboid, and that each one contains a central body of
different staining properties from the remainder of the structure. This
has been regarded by some as a nucleus. On being brought into contact
with a foreign surface they adhere to it firmly, very rapidly passing
through a number of phases resulting ultimately in the formation of
granular debris. In shed blood they tend to collect into groups, and
during clotting, fibrin filaments may be observed to shoot out from
these clumps.

_Variations in the Blood of different Animals._--If we contrast the
blood of different animals of the vertebrate class we find striking
differences both in microscopic appearances and in chemical properties.
In the first place, the corpuscles vary in amount and in kind. Thus,
whilst in a mammal the corpuscles form 40 to 50% of the total volume of
the blood, in the lower vertebrates the volume is much less, e.g. in
frogs as low as 25% and in fishes even lower. The deficiency is chiefly
in the red corpuscles, the ratio of white to red increasing as we
examine the blood from animals lower in the scale. The corpuscles
themselves are also found to vary, especially the red o