THE VITAL PROPERTIES OF THE CELL 



67 



flow back again, thus causing them to 

 decrease in size, until finally they are com- 

 pletely withdrawn, whilst new processes 

 are being protruded from another portion 

 of the body. By means of these alternate 

 protrusions and retractions of their pseudo- 

 podia, the small bodies of protoplasm are 

 enabled to move from place to place, crawl- 

 ing over the objects to whose surfaces they 

 cling at a rate which can only be measured 

 under the microscope. Amosbce are able to 

 traverse a distance of h mm. in a minute. 



In this manner the white blood cor- 

 puscles during inflammation are able to 

 pass through the walls of the capillaries 

 and of the smaller vessels, and the lymph 

 corpuscles make their way as w T andering 

 cells into the connective tissue spaces, such 

 as the interlamellar spaces of the cornea, 

 where the resistance to be overcome is not 

 great, or they force their way between 



epithelial 



Fig. 37. A Leucocyte of 

 the Frog containing: a Bac- 

 terium which is undergoing 

 the process of digestion. 

 The Bacterium, has been 

 stained with vesuvine. The 

 two figures represent two 

 successive changes of shape 

 in the same cell. (After 

 Metschnikoff, Fig. 51.) 



Fig. 38. Amceha proteus (after Leidy ; 

 from R. Hertwig, Fig. 1G ) : n nucleus; 

 cv contractile vacuole; N food vacuoles; 

 en endoplasin; elc ectoplasm. 



cells, and 

 so reach 

 the sur- 

 face of an epithelial membrane. 



This extension and retraction 

 of pseudopodia is most marked in 

 a small Amoeba (Fig. 38), which 

 was described as far back as 1755 

 by Roesel von Rosenhof, who on 

 account of its energetic changes 

 of form called it the small Fro- 

 tcus. 



Somewhat different movements 

 take place in Myxomycetes, and 

 in Thalamophora, Heliozoa, and 

 Radiolaria. 



The plasmodia of some species 

 of Myxomycetes, such as the 

 JEtlxalium septicum, often spread 



