384 K. M. SMITH 



E. The Ultimate Infective Unit 



The criticism has been made of a too facile assumption that the various 

 types of particles associated with the different insect virus diseases are the 

 actual virus particles. In the writer's opinion this criticism is not valid, in 

 view of the extreme similarity between these particles and other viruses of 

 proved infectious nature. So far as the nuclear polyhedroses are concerned 

 the criticism does not apply, since Bergold (1953) has demonstrated the high 

 degree of infectivity possessed by the purified rods extracted from the poly- 

 hedra. It is true that the parallel experiment with the purified particles from 

 the cytoplasmic polyhedra does not seem to have yet been done. It is known, 

 however, that the polyhedra are extremely infectious; a glance at Figs. 19 

 and 20 will show how exactly similar the extracted particles are to other 

 viruses. It is perhaps more legitimate to inquire what the ultimate infective 

 unit is and how far the numerous enclosing capsules and membranes influence 

 the infectivity of some viruses. 



In certain viruses from cytoplasmic polyhedroses the particle appears to be 

 composite and at very high magnification, a number of apparent subunits are 

 visible (Fig. 11). Experimental evidence as to whether these subunits are 

 infectious has not yet been obtained but they are interesting as affording a 

 possible insight into the build-up of some spherical infectious particles. 



IV. Pathological Changes in the Infected Cell and 

 the Development of the Virus Particles 



In the nuclear polyhedroses the most striking pathological change is the 

 development of the central chromatic mass or net, referred to by Xeros 

 (1956) as a "virogenic stroma." This central mass has been observed also by 

 earlier workers, but the first electron micrograph of a section through this 

 nuclear net was shown in a polyhedrosis of the larva of the privet hawk moth, 

 Sphinx ligustri (Smith et al., 1953). The chromatic mass or net was once 

 thought to be a stage in the further development of a fused aggregation of 

 nucleoli (Mazzocchi, 1908; Glaser, 1927). Another view was that the net was 

 formed primarily by the fusion of chromatin granules of the pathological 

 nucleus (Paillot, 1926c; Heidrenreich, 1940). However, according to Xeros 

 (1955), who made a detailed histological study of a number of nuclear 

 polyhedroses, no evidence was found that the chromatic mass had been 

 formed from the chromatin. He concludes that the chromatic masses or nets 

 do not arise by fusion of chromatin granules but are produced de novo. It 

 seems clear that this chromatic mass is the site of the development of the virus 

 rods, but about the exact method of development of the virus rods there is 

 still a good deal of uncertainty. Bergold (1950, 1953) postulated a life cycle 



