110 PROTISTS AND DISEASE 



stage, this virus also will be filtered, indeed one positive 

 result has already been recorded by Jahnke. 



Remlinger used immediate rapid filtration through a 

 Berkefeld 'V.' In prolonged filtration this filter becomes 

 imperfect. Fixed virus obtained from the brains of rabbits 

 the spinal cords of which had been used in antirabic treat- 

 ment was the material he employed. We may best appreciate 

 the facts connected with filtration of living organisms by 

 studying some of the records. A method used for molluscum 

 is given below in this Chapter. 



Sheep-pox virus Borrel got by scraping the under surface 

 of skin removed from a large pustule 5 to 6 cm. across. 

 This mixed with 100 c.c. of tap- water proved to be virulent 

 after filtration through a Berkefeld filter, the filtrate giving 

 no growth on the usual media. Rous found a transplantable 

 spindle-celled sarcoma in chickens ; it had a filtrable virus, 

 which reproduced similar tumours in fowls ; and when 

 injected into eggs, on the membranes, and sometimes on 

 the body of the chick. 



The Size of Microhenads. Speculations on this subject 

 have been numerous ; K. T. Hewlett has given us data by 

 which we can form a mental picture on the supposition that 

 microhenads are rigid solid bodies : " Visually, when an 

 object comes into the region of a little under one fifth of a 

 micron you cannot see it, even though you use the very 

 best lenses. Although an object of such dimensions is very 

 small, compared with molecules it is of course very large ; 



