52 OUTLINES OF EVOLUTIONAEY BIOLOGY 



If these gain entrance into the tissues at any weak spot they are 

 set upon by the leucocytes and literally devoured. This process 

 is known as phagocytosis, and in this capacity the leucocytes are 

 often spoken of as phagocytes. It is obvious that the health of 

 the body must depend largely upon the activity of the phagocytes 

 and their efficiency in dealing with disease-producing " germs." ^ 

 The white blood-corpuscles, then, differ in no essential parti- 

 cular as regards their structure and mode of life from so many 

 Protozoa. It is true they cannot live permanently outside the 



: v, 

 



FIG. 15. Blood Corpuscles of the Frog, X 326. (From a photograph.) 



a., white corpuscle or leucocyte; b., red corpuscles or hsematids. The nuclei appear 

 light-coloured in the photograph owing to their having been stained blue in the 

 preparation. 



body, but that is also the case with many parasitic Protozoa 

 which live in the blood of other animals. That they are not 

 independent organisms, but form an integral part of the body 

 in which they occur, is, however, obvious from the fact that 

 they have a common origin with all the other tissues from the 

 developing ovum. 



The red corpuscles, or hsematids, are very different bodies. 

 They float passively in the blood-stream and serve as the carriers of 

 oxygen gas from the respiratory organs to the various tissues. 

 Unlike the leucocytes they have definite and constant outlines, 

 though, owing to the flexible nature of the thin cell-membrane by 

 which they are enclosed, they may undergo temporary distortion. 



