PHAGOCYTOSIS 161 



The fact that several species of fossil bacteria are known from the 

 carboniferous and other periods (144) shows that they are among the oldest 

 inhabitants of the earth, and were already widely distributed when the 

 Tertiary Epoch and the evolution of warm-blooded creatures began. We 

 should therefore expect that these would, in the course of their phylo- 

 genetical progress, have gradually developed the power of resisting the 

 attacks of the invading micro-organisms. 



The investigation of these protective properties is the subject of ex- 

 tensive research and has already, in the field of sero-therapeutics, borne 

 fruit that promises much for the future, although it is true that at present 

 experience and theory are opposed to each other on many points, and 

 a long time must elapse before a reliable foundation is reached. 



The first of the defensive appliances of the body to which attention 

 was directed (mainly by Metschnikoflf and his followers) were the white 

 blood-corpuscles or leucocytes, the important part played by which in 

 physiological and pathological processes has come to be more and more 

 recognized. These naked cells that 

 have their origin in the spleen, the 

 bone-marrow, and other haemopoeic 

 organs of the body, are often called 

 ' wandering cells,' because they are 

 able to leave the blood-vessels and 



FlG. 29. Phagocytosis, from Metschnikoff. a, 

 lymphatlCS in Which they are distri- leucocyte from pigeon's blood with anthrax bacilli. 



These are in some cases intact and stain well 

 buted OVer the Whole body and Dene- (black), in others degenerate and pale. The oval 



body is the cell nucleus. 6, a living leucocyte from 

 tratC between the fixed Cells Of the a pigeon, devouring anthrax bacilli. 



tissues. They are found, for instance, 



actively multiplying in the intestinal villi during digestion, and as 'pus- 

 corpuscles ' they constitute the chief part of pus. An increase of them in 

 the blood-vessels is spoken of as leucocytosis. 



Now in the blood of animals infected with anthrax, and in suppurative 

 processes of all kinds, the leucocytes frequently (although not always) con- 

 tain bacteria. These are sometimes present in great numbers, and often 

 appear to be dead, inasmuch as the cell-contents are changed and stain 

 badly (Fig. 29, a and b}. On these phenomena Metschnikoff founded his 

 theory of Phagocytosis (145). The wandering cells were supposed to 

 function as phagocytes (devouring cells), to take up and to kill any bacteria 

 that had penetrated into the tissues. They were supposed, in fact, to 

 constitute an army of defence that could be mobilized and sent to any 

 part of the body. This view, the result of direct observations, was further 

 strengthened when the chemotactic properties of leucocytes (153) were 

 studied, and, above all, their behaviour towards bacteria and towards the 

 products of their metabolism contained in pure cultures. 



Capillary tubes filled with bacterial substances, introduced into the 



FISCHER M 



