98 
COMPARATIVE ZOOLOGY. 
ently a clear, homogeneous liquid, really consists of minute 
grains, or globules, of organic matter floating in a fluid. 
If the blood of a Frog 
be poured on a filter of 
blotting-paper, a trans¬ 
parent fluid (called plas¬ 
ma) will pass through, 
leaving red particles, re- 
h sembling sand, on the 
upper surface. Under 
| the microscope, these 
particles prove to be 
cells, or flattened disks 
(called corpuscles), con- 
b 
Fig. 62.—Red Blood-corpuscles of Man : a, shows 
circular contour; 6, a biconcave section; c, a 
group in chains. 
c, a taming a nucleus; some 
are colorless, and others 
red. The red disks have a tendency to collect together 
into piles; the colorless ones remain single. Meanwhile, 
the plasma separates into two parts by coagulating; that 
is, minute fibres form, consisting of fibrin, leaving a pale 
yellowish fluid, called serum . 55 Had the blood not been 
filtered, the corpuscles and fibrin would have mingled, 
forming a jelly-like mass, known as clot . Further, the 
serum will coagulate if heated, dividing into hardened 
albumen and a watery fluid, called serosity, which contains 
the soluble salts of the blood. 
These several parts may be expressed thus: 
Blood 
albumen. 
serosity=water and salts. 
If now we examine the nutritive fluid of the simplest 
animals, we find only a watery fluid containing granules. 
In Radiates and the Worms and Mollusks, there is a similar 
fluid, with the addition of a few colorless corpuscles. But 
