THE WHITE BLOOD CORPUSCLES 203 



end removing from it the red corpuscles by means of a lateral drainage 

 stream of slight force. The white cells then stick to the surface of the 

 slide and, if kept in a warm isotonic solution, may be studied for some 

 time thereafter. They may also be obtained from the frog by insert- 

 ing a platelet of porous wood under the skin covering the dorsal aspect 

 of its body. If permitted to remain in this position for several 

 hours, the meshes of the wood will be filled with many leukocytes, the 

 removal of which can easily be effected by washings with normal saline 

 solution. They may be studied in a more plastic manner by placing 

 the frog's mesentery or bladder under the microscope. In the cir- 

 culating blood they appear as translucent globular bodies, which, on 

 account of their lesser specific gravity, leave the swift axial stream and 

 enter the more slowly moving peripheral layers of the current. They 

 attach themselves here or there to the vessel wall, but soon pass on- 

 ward again by executing a peculiar rotary motion. 



Under favorable conditions the leukocytes exhibit a movement 

 of their cytoplasm^ which is very similar to that displayed by the 

 ameba. Their substance contracts and relaxes alternately, while their 

 nuclear constituents remain rather stationary and serve, so to speak, 

 as a center for this movement. Prolongations, commonly designated 

 as pseudopodia, are sent out in different directions into the surrounding 

 medium to be again retracted later on with varying swiftness.^ Thus, 

 a leukocyte may extend and retract its pseudopodia repeatedly without 

 altering its position, but it may also happen that one of its prolonga- 

 tions becomes attached to the surface and that the remaining mass of 

 the cell is slowly moved onward in the direction of this fixed point. 

 This propertj^ of the leukocytes to adhere to surfaces is attributed by 

 Verworn to the extrusion of a mucous secretion. When freely moving 

 they usually present a globular outline which implies that they are in 

 a state of contraction. 



Phagocytosis. — Whether the leukocyte remains stationary or 

 moves onward to a different place, the molecular shifting of its sub- 

 stance is instrumental in bringing it into relation with various particles 

 of food and other extraneous material. As is true of other low forms 

 of life, the leukocyte behaves in a very characteristic manner toward 

 these substances, being either attracted or repelled by them. This 

 orientation is brought about largely by chemical means, and hence, the 

 leukocytes may be said to possess the property of chemotropism or 

 chemotaxis of a positive and negative kind. 



The chemotropic qualities of the leukocytes must be held responsible 

 for their power of taking up nutritive particles and of englobing and 

 digesting all that material which is foreign or injurious to the body. 



^ First observed by Wharton Jones in 1846, and proved for the human leuko- 

 cyte by Davaine in 1850. Lieberkiihn gave an adequate description of this 

 movement in 1854. 



2 Verworn, Pfliiger's Archiv, li, 1891; also see: Maximow, Ziegler's Beitrage, 

 Ixxiii, 1909 and Ixxvi, 1910. 



