386 K. M. SMITH 



are minute polyhedral crystals. It would be extremely difficult to postulate 

 any form of developmental cycle for particles of this nature. 



Xeros (1956) considers that the virus particles of cytoplasmic polyhedroses 

 arise in "virogenic stromata" essentially similar to those of the nuclear 

 polyhedroses. He has studied a cytoplasmic polyhedrosis in a processionary 

 caterpillar or "army worm," Thaumatopoea pityocampa, in which he describes 

 virogenic stromata with a micro-net structure. In the cords of the stromata 

 virus bodies arise which are spherical, with an extremely dense center about 

 35 nut in diameter and a less electron-dense cortex about 80 nux in diameter. 

 When the virus bodies have been formed, the cord material around them 

 disrupts and liberates them into the larger pores formed as a result of dissolu- 

 tion of the cords. The freed virus bodies become occluded in the polyhedra. 



V. Latent Infection in Insects and its Bearing on 

 the Cross-Transmission of Viruses 



There is a good deal of confusion caused in the study of latent infections by 

 a lack of uniformity in the use of such words as "latent," "inapparent," 

 "subclinical," and "masked." A symposium on the subject was recently held 

 at Madison, Wisconsin, and the chief findings were reported by Andrewes 

 (1957). It was suggested that the words "latent virus" should not be used but 

 that "inapparent infection" at the host-parasite level would cover the whole 

 field of infections which give no overt signs of their presence. "Latent infec- 

 tion" denotes those cases of inapparent infection which are chronic and in 

 which a certain host-virus equilibrium is established. At the cell-virus level, 

 Dulbecco's term "moderate virus" was approved to denote a virus which 

 grew in a cell while still permitting its continued survival and multiplication; 

 "cytocidal" described one which killed it; "sub moderate" covered inter- 

 mediate cases. 



There is no doubt that large populations of insects, particularly lepidop- 

 terous larvae, carry latent infections; this fact is liable to cause much 

 confusion in experiments in the cross-transmission of insect viruses. 



For many years the opinion was held that insect viruses were extremely 

 specific and that true examples of transmission between different species 

 were unknown; examples of apparent cross-infection in the literature were 

 regarded as unproved. In 1953 a paper was published (Smith and Xeros, 

 1953b) in which a large number of experiments, involving apparently genuine 

 cross-transmission of viruses, was described. The alternative possibility of 

 stimulation of latent infection in some cases was, however, admitted. Up to 

 that date the term "insect virus" referred mainly to the nuclear polyhedral 

 viruses and possibly to those of the granuloses. With the discovery of the 

 distinct group of cytoplasmic polyhedroses the situation became more 

 complicated. 



