98 



COMPARATIVE Z00L03Y. 



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^fes- 

 ma) will pass through, 

 leaving red particles, re- 



sembling sand, on the 

 upper surface. Under 

 the microscope, these 

 particles prove to be 

 cells, or flattened disks 

 (called corpuscles), con- 

 taining a nucleus; some 

 are colorless, and others 



Pio. 62 Rerl Blciod-corpnecles of Man : a, shows 



circuhir conioni"; 6, a bicoucave section; c, u, 

 group lu cliaiua. 



red. The red disks Rave 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 ^JWm, leaving a pale 

 yellowish fluid, called serum.^ 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 

 tlie soluble salts of the blood. 



These several parts may be expressed thus : 



Blood 



„ , (colored ) 



Corpuscles t , , J— 

 I colorless) 



! fibrin- 

 serum 



albumen. 



Plasma \ 



( 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 MoUusks, there is a similar 

 fluid, with the addition of a few colorless corpuscles. But 



