THE ILEMOFLAGELLATES AND ALLIED FORMS 317 



are probably taken up by leucocytes, and in them they may be 

 carried into the general circulation. 



Although the diseases caused by these parasites are termed 

 comprehensively "' leishmanioses," they are not all of one type. 

 L. donovani produces a systemic disease, very deadly in its effects, 

 and the parasite is found in immense numbers in the spleen, bone- 

 marrow, liver, etc. L. infantum is similar in its effects. L. tropica, 

 on the other hand, produces a purely local infection, manifested 

 in the form of one or more boils on the skin, each of which, accord- 

 ing to Wenyon (84), represents either a single infection by the 

 insect, as yet not known with certainty, which transmits the 

 disease, or a secondary infection by a house-fly or by the in- 

 dividual himself from another boil on the skin. The infection by 

 L. tropica has an incubation-period of about two months. The 

 disease lasts from twelve to eighteen months, and one attack, after 

 it is healed spontaneously, confers absolute immunity for the 

 rest of the patient's life. Corresponding with these differences in 



A B C D E 



PIG. 139. Leishmania donovani. A, Three parasites in the ordinary condition, 

 each showing a larger trophonucleus and a smaller kinetonucleus ; B, C, D, 

 stages of binary fission ; E, multiple fission into three parts. After Chris- 

 tophers. 



the effects produced by the parasites, there are also slight structural 

 differences to be made out in them. L. donovani (Fig. 138) is very 

 uniform in shape, being rounded or ovoid ; L. tropica (Fig. 140), 

 on the other hand, shows more variety of form, with every transition 

 from elongated, narrow forms with one end pointed to the typical 

 oval body (Row, Wenyon). 



No other stage than that described above is known from the 

 human body ; but it was first discovered by Rogers (576) for 

 L. donovani, and subsequently confirmed by other investigators 

 for this and other species, that in artificial cultures the parasite 

 develops into a typical leptornonad form (Fig. 140). The Leish- 

 mania-foims in the cultures grow considerably in size, and at the 

 same time multiply by fission. The relatively large rounded forms 

 become pear-shaped, and a flagellum is developed at the blunt end 

 of the body ; finally the organism assumes the typical elongated form 

 of a leptornonad, with a long flagellum arising close beside the kine- 

 tonucleus, which is situated near the anterior end of the body. 

 Leishman and Statham have described a further stage in the 



