136 MICROBES AND TOXINS 



the primitive amceboid type. They are in general the least 

 differentiated elements in the body, but they are also the most 

 independent and possess the greatest vitality. They assist in 

 building up the young animal during the embryonic period, and 

 when the tissues begin to wear out, when old age is coming on, 

 it is the phagocytes which consume the senile cells incapable 

 of recovery and take their place. The renewal of cells and 

 tissues which goes on slowly and continuously in a great many 

 animals is, like the abrupt transformation which occurs in 

 metamorphosis, the work of phagocytes." 



The importance of pathological phagocytosis from the medi- 

 cal point of view should not make us forget that a normal 

 phagocytosis exists. The histolysis in the larvae of insects, the 

 destruction of the tail in the tadpole forms of the Tunicata, the 

 degeneration of the tail muscles in the tadpole of the Frog and 

 Toad, the destruction of the myelinated nerve fibres in Wallerian 

 degeneration, the shrinking of the ovarian follicles, the fixation 

 of the ovum on the mucous membrane of the uterus, the daily 

 destruction of the red corpuscles of the blood which goes on 

 in the spleen, all are examples of normal phagocytosis. 



The phagocytes are guided or directed in their choice and 

 perception of the bodies which they ingest, by a peculiar sense 

 whose manifestations are known by the name of chemiotaxis. 



It has been known since the time of Pfeffer and Stahl that 

 cellular organisms and plasmodia are attracted by certain 

 substances (positive chemiotaxis), and repelled by others 

 (negative chemiotaxis). They become accustomed to sub- 

 stances which at first repelled them and finally are attracted by 

 these. 'Massart and Ch. Bordet have systematically studied 

 chemiotactic actions, by introducing under the skin of the frog 

 capillary tubes containing chemical substances, microbes and 

 their products. Lactic acid, glycerine, bile, and guanin repel 

 the leucocytes ; sterilised cultures of both saprophytic and 

 pathogenic microbes attract them. Positive chemiotaxis may 

 be considered as the appetite, which prepares for intracellular 

 digestion. Everything is not yet explained in this distant 

 action. Chemiotaxis is analogous to the sensations of higher 



