98 
COMPARATIVE ZOOLOGY. 
ently a clear, homogeneous fluid, really consists of minute 
grains, or globules, of organic matter floating in water. 
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- 
*0 sembling sand, on the 
upper surface. Under 
| the microscope, these 
particles prove to be 
cells, or flattened disks 
(called corpuscles ), con- 
c 
b ^b 
Fio. 62.— Red Blood-corpnecles of Man: a, shows ^ 
circular contour; b, a biconcave section; c, a taining a nucleus ; SOine 
group in chains. ■, , 
are colorless, and others 
group in chains. 
red. The red disks have a tendency to run together into 
piles; the colorless ones remain single. Meanwhile, the 
plasma separates into two parts by coagulating; that is, 
minute fibres form, consisting of fibrine , leaving a pale 
yellowish fluid, called serum.™ Had the blood not been 
filtered, the corpuscles and fibrine 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: 
Llood 
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 sim¬ 
ilar fluid, with the addition of a few white corpuscles. But 
