MANUAL OF ELEMENTARY ZOOLOGY 



without nuclei. The white corpuscles or leucocytes are 

 colourless and smaller than the red. They have not, like 

 the red, a definite shape, but consist of very soft undiffer- 

 entiated protoplasm, which has kept its power of contraction, 

 and when the corpuscle lies against a solid surface is con- 

 stantly changing its shape, putting forth in all directions 

 irregular processes or " pseudopodia " and as readily with- 

 drawing them again. By continually lengthening a pseudo- 

 podium and withdrawing those on the opposite side they 

 can flow along. Movement of this kind 

 is called amozboid because it occurs in a 

 minute animal known as Amceba, which 

 we shall presently study. In the blood 

 stream the leucocytes take on a rounded 

 shape, so that they are easily bowled 

 along. The white corpuscles are of 

 several kinds. Some of them are of use 

 to the organism by flowing round and 

 thus engulfing into their protoplasm 

 harmful bacteria, which they digest. 

 Such are known as phagocytes. At times 

 several phagocytes flow together to form 

 a common mass of cytoplasm containing 

 several nuclei, known as a macrophage. 

 Other leucocytes secrete substances 

 which either destroy bacteria or neutral- 

 ise the poisons that the latter secrete. 

 Leucocytes also carry substances, such as 

 fat globules, from one place to another. 



When blood is shed it clots, owing 

 to the precipitation in the plasma of a 

 protein known as fibrin, in the form of a 

 meshwork of fine fibrils which entangles 

 the corpuscles and forms a firm mass. Fibrin is formed 

 by the union of the protein fibrinogen with a small quantity 

 of another substance thrombin, under the influence of a sub- 

 stance present in the tissues which are wounded when the 

 blood is shed. The liquid which remains after the forma- 

 tion of the clot is known as serum. The effect of .clotting 

 is to close wounds and thus prevent loss of blood. 



The white corpuscles are semi-independent portions of 



Fig. 59. — A portion 

 of a striped muscle 

 fibre, magnified. 



