ECOLOGY, COMMENSALISM AND PARASITISM 367 



ally used under the designation NNN agar medium. In this medium 

 the same strains of Leishmania have been (1925) maintained for up- 

 ward of fourteen years with hundreds of sub-cultures (Nicolle, 1925). 



In a nutrient medium— digestive tract, blood, cell or artificial 

 medium— the normal, fully-developed flagellates reproduce by longi- 

 tudinal division. Blepharoplast and parabasal body are the first to 

 divide, then the nucleus in which the endobasal body initiates divi- 

 sion (Fig. 1(39, B and F). In some cases the old flagellum is retained 

 by one of the daughter cells, a new flagellum growing out from the 

 blepharoplast in the other cell. Multiple division of -kinetic ele- 

 ments and nuclei, without accompanying cell division— so-called 

 somatella formation— is characteristic of some types, particularly 

 during intracellular stages, e.g., Trypanosoma lewisi (Fig. 122, p. 234). 



Reproduction by division is not confined to the fully-developed 

 flagellates but may occur in any phase. Thus the " crithidia forms " 

 or haptomonads of Trypanosoma may divide while attached to 

 host cells as do the Leishmania forms within cells. " Leptomonas 

 forms" (nectomonads) likewise divide. 



The genus Leptomonas is the simplest of this family of parasitic 

 flagellates. It is represented by many species which are widely 

 distributed amongst insects and by one species in nematode worms 

 (L. biitschlii, Kent, in Triloba gracilis). Encystment occurs in the 

 digestive tract, the cysts passing out with the feces and infection 

 is contaminative. Only one host— invertebrate— is known. 



Structural changes are simple, from the fully-developed necto- 

 monad with kinetic complex anterior to the nucleus, and long 

 flagellum, it becomes progressively shorter and loses its flagellum. 

 In this condition it may become attached to epithelial cells of the 

 gut and Malpighian tubes (haptomonads) or it may become still 

 smaller, develop a protecting covering and pass out with the feces. 



Crithidia is a second genus of the family with only one host 

 (invertebrate) and causing infection by contamination through the 

 agency of cysts. It also is widely distributed amongst the insects 

 and particularly in Diptera. Structurally it is similar to Lepto- 

 monas with the kinetic complex anterior to the nucleus. The endo- 

 plasmic portion of the axial filament, however, passes to the margin 

 of the body and continues along that margin until it leaves the 

 body at the anterior end, thus giving the impression of a rudimentary 

 undulating membrane (Fig. 170). As in Leptomonas the swimming 

 nectomonad becomes progressively shorter, attaches by the flagellar 

 end to epithelial cells where it may reproduce by longitudinal divi- 

 sion. Large areas of the exposed surface of epithelial cells may be 

 covered in this manner thus hampering the functional activity of 

 these cells (Fig. 170, F). 



Leishmania shows an interesting and important step in progres- 

 sive parasitism leading to serious, often fatal, diseases of man and 



