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 (caWedjplas- 

 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 

 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 : 



Fig. 62.— Red Blood-corpnscles of Man : a, shows 

 circular contour; 5, a biconcave section; c, a 

 group in chains. 



Blood 



( Corpuscles ^ co J 01 ^ I- 

 J (.colorless) 



( Plasma Jfi Dime , 

 I serum -j 



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 



