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Proceedings of the Royal Physical Society. 



occurs in quite a relatively early stage of digestion. In the 

 intestine they develop the locomotor apparatus, but persist 

 for a considerable period in the form of a Herpetomonas-like 

 individual (Figs. 28-41), with a short, straight flagellum, the 

 development of which will be traced later. Division (Figs. 

 43-49) occurs actively amongst these stages. They vary 

 considerably in size, and are often very small. It is to be 

 noted that these Herpetomonas forms are very persistent, 

 and are to be found through the whole period of infection in 

 greater or less numbers. As time goes on, these animals 

 develop into trypanosomes, which show much variation in 

 size and appearance, and particularly in the position of the 

 trophonucleus and kinetonucleus. In fact, these stages show 

 a quite remarkable polymorphism (Figs. 16-27). 



The trypanosomes are, as a rule, in the intestine, but are to 

 be met with in the crop, and also in the proboscis, often in 

 large numbers. At about the middle period of digestion the 

 intestine of a well-infected animal presents a most astonish- 

 ing number of forms. Herpetomonas stages in every grade 

 of development, and trypanosomes varying from broad indi- 

 viduals with a relatively slow movement through space, to 

 long, slender individuals, which move as rapidly, and after 

 much the same fashion, as Spirochceta anodontcB. 



Finally, towards the end of digestion, only long, slender 

 trypanosomes and a few Herpetomonas stages, which are 

 obviously proceeding directly to this type, are to be seen. 

 Amongst these Herpetomonas individuals are always a number 

 of very early stages, which may be quite small. It is these 

 slender trypanosomes which migrate up into the proboscis, 

 where they may become still more slender, until they are 

 thread-like creatures, with hardly any protoplasm (Figs. 26 

 and 27). 



At the close of digestion, that is to say, when the crop is 

 quite empty of blood, and when the intestine shows none of 

 the dark fluid formed by the breaking down of the blood by 

 the digestive fluids of the leech, there may be very large 

 numbers of the long slender trypanosomes pretty well 

 throughout the whole leech, but very often they are still 

 most numerous in the intestine. Besides these, there are 



