HISTOLOGY OF THE FROG 75 



scarcely affected. Some of the leucocytes contain only a 

 single nucleus, but, as a rule, they have two, three, or even four 

 contained in a single protoplasmic envelope. The body of 

 the leucocyte has no definite outer limiting membrane, and no 

 constant shape. It usually contains granules of different 

 kinds, and a few clear spaces called vacuoles, filled with fluid. 

 Simple as its structure is, the leucocyte is living, endowed 

 with the power of spontaneous movement, and capable of ex- 

 hibiting by itself some of the fundamental properties of living 

 organisms. In a sense a leucocyte may be said to be an 

 independent organism, but its independence is not complete. 

 It is incapable of separate free existence apart from the blood 

 which forms its normal environment, and therefore we cannot 

 isolate leucocytes, and study them apart from the blood, and 

 deal with them as independent living beings. But within the 

 .blood as it courses through the tissues, or in blood freshly 

 drawn from the body, the leucocyte behaves as a separate 

 organism complete in itself. That it is contractile, and capable 

 of change of shape, is easily seen on simple inspection of a 

 sample of freshly-drawn blood. The leucocytes do not remain 

 still, nor do they retain a constant form as do the hsematids, 

 but are continually in movement, putting forth irregular blunt 

 processes of their protoplasm in various directions, and again 

 withdrawing them. These blunt movable processes are called 

 pseudopodia, and the movement due to them, from its resem- 

 blance to that of a minute organism, the Amoeba, which we 

 shall study presently, is called amoeboid. That the leucocytes 

 are "automatic" that is to say, endowed with the power of 

 spontaneous movement is shown by their constant changes of 

 form ; changes which, so far as can be determined, are called 

 forth by no definite external stimulus. But the corpuscle is 

 also irritable; for if the blood be very gently warmed the 

 activity of the pseudopodial movement is increased ; if the 

 temperature be much lowered it will be decreased or stopped 

 altogether, to recover when the temperature is raised again. 

 Certain drugs also, such as quinine, retard the activity of the 

 leucocytes. The leucocytes are assimilative, for they surround 

 and take into their substance foreign substances; many of 

 them are seen to be filled with fat granules, and it has been 

 shown that these are ingested, swallowed, as it were, during the 

 passage of the blood through the intestine. And leucocytes 



