SIZE, SHAPE, AND HYDRATION OF VIRUSES 21 



bodies are observed in the hair cells of Nicotiana tabacum, and 

 these are infective (Bawden, 1950, p, 44), these observations 

 are a confirmation of the extensive internal cellular growth of a 

 plant virus. 



Very interesting information about the nature of influenza 

 virus and of Newcastle disease virus has been obtained by this 

 method. Wyckoff (1951) has examined sections of the chorio- 

 allantoic membrane of chick embryos infected with PR-8 in- 

 fluenza virus. He found filaments extending outward from the 

 membrane with spherical particles apparently associated with, 

 and perhaps being capable of development from, the filaments. 

 Since the appearance of ultracentrifugally "purified" virus 

 shows spherical particles (which are infectious), the question 

 is raised as to whether the spherical particles are actually in 

 spherical form during the process of multiplication. Similar 

 results were found for Newcastle disease by Kilham, Morgan, 

 and Wyckoff (1951). Filaments growing out into the chorio- 

 allantoic fluid were observed with some spherical particles 

 possibly associated with them. 



Very beautiful })ictures of the curious mosaic of vaccinia in 

 the same chick-embryo membrane have been obtained by 

 Wyckoff (1951). Again, the striking feature of the pictures is 

 the great extent to which the cytoplasm is manufactured into 

 virus particles. 



The effect of the combination between two spherical plant 

 viruses and specific antiserum has been studied by Black, 

 Price, and Wyckoff (1946). The virus particles can be seen to 

 be in microflocculent aggregations with about twice their normal 

 separation. 



A very important series of electron-micrograph ic studies of 

 plant viruses in unpurified extracts has been made by Bawden 

 and Nixon (1951). In the following cases they found no specific 

 particles: tomato spotted wilt, potato leaf roll, sugar beet 

 mosaic, sugar beet yellows, cauliflower mosaic, and tomato 

 aspermy virus. They could not have detected less than 10 mg/1 

 if spheric shaped, or less than 1 mg/1 if rod shaped. For potato 

 virus X, they observed rod-like, tenuous particles 100 A wide 



