102 BLOOD AND LYMPH. 



the parasite and drawing itself up behind, it advances with a 

 mode of locomotion resembling that of a snail. Finally, it 

 sends processes around the parasite and engulfs the latter in 

 its body, where it is disintegrated. The leukocyte seems en- 

 dowed with intelligence, purpose, and a separate individuality 

 of its own, vividly analogous to the psychic manifestations of 

 macroscopic animals. 



Chemical substances are supposed to have an influence on 

 leukocytes, some attracting them, others repelling them ; this 

 is called chemotaxis. Positive chemotaxis consists in the attrac- 

 tion of leukocytes toward certain substances, negative chemo- 

 taxis in their repulsion from other substances. 



The functions of leukocytes are not entirely understood. 

 One important purpose which they are believed to subserve 

 is the removal and destruction of foreign noxious substances 

 and parasites, by phagocytosis; they also play an active part 

 in the repair of injuries, in immunity, and the resistance and 

 reaction of the body to morbid processes. 



Origin and development of leukocytes : The small mononu- 

 dear leukocytes are doubtless derived from the free lymphoid 

 cells which crowd adenoid tissues ; these undergo development, 

 enter the lymph-stream, and are thence carried into the blood. 



The large mononuclear and transitional leukocytes appear to 

 develop from the small mononuclears, as intermediate grada- 

 tions between them are to be found in the blood. 



The source of the polymorphonuclear and eosinophile leuko- 

 cytes is not so obvious. They may develop from the mononu- 

 clear variety or, as has been suggested, the eosinophiles 

 (" old " forms) may develop from the polynuclears, and the 

 latter ("adult" forms) from the mononuclears (" young " 

 forms) ; as, however, there are no intermediate forms between 

 these varieties in the circulating blood, their development 

 from one another must take place, if at all, somewhere in the 

 viscera. Or perhaps the polynuclear and eosinophile leuko- 

 cytes originate independently in some of the viscera, as the 

 spleen or bone-marrow. 



The eosinophiles, especially, may not improbably develop 

 in the marrow, as in the latter are found similar cells contain- 

 ing oxyphile granules. Division of the developed leukocytes 

 themselves may be a partial means of their production. 



