126 Society for Experimental Biology and Medicine. 



one half turns, being 3.55 a long. Rather infrequently spirals 3.75 

 to 4.00 /jl in length are seen, and these present central constrictions 

 suggesting transverse division. From the striking variation in 

 length also, this would appear to be the mode of multiplication. 

 All intermediate lengths are seen. The width of the filament is 

 approximately 0.25 fi, the gross width of the coil about 0.65 //. 

 The length of a turn or node (crest to crest) is 1.0 to 

 fairly constant, but indicating some longitudinal extensibility of 

 the coil. By raising the plane of focus above the stained parasite 

 the upper segments of the turns are seen more clearly than the 

 lower. They run forward from left to right. In lower focus the 

 lower halves are the more clearly defined, and are seen to extend 

 forward from right to left. The spiral therefore corresponds to 

 the ordinary right hand screw, turning clockwise as it proceeds 

 from a given point. This seemed to be true of all the individuals 

 examined, though some were too much flattened against the glass 

 to manifest an appreciable difference in focus. 



The infection is readily transferred to other wild rats by intra- 

 peritoneal injection of a very small drop of infected blood in 

 normal salt solution. In many cases not more than ten or twenty 

 parasites could have been present in the injection, yet, so far, the 

 wild rats have always developed the infection. Of seven wild rats 

 inoculated, the parasites were first detected in the blood of one on 

 the fifteenth day, in one on the eleventh, in one on the twelfth, in 

 three on the tenth and in one on the seventh day. The last 

 mentioned was a small rat and received a relatively large dose, one 

 fourth cubic centimeter of defibrinated blood and one-half cubic 

 centimeter of a thick suspension of the organs in salt solution, 

 from a rat showing one spirochete in every four fields of blood 

 film. The parasites never become very numerous and disappear 

 in from one to nine days. This apparent recovery is then followed 

 by repeated relapses. The parasites may become more numerous 

 in the blood during the relapse than in the primary invasion. 

 Neither a certain recovery nor a fatal result has, as yet, been 

 i -bserved. 



White rats are susceptible, with an incubation period of four 

 to eight days, according to the dose employed. 



The house mouse [Mus musculus) is apparently more resistant. 



