THE MORPHOLOGICAL APPROACH 79 



B. Fowlpox 



1. Inclusion 



The fowlpox inclusion is considered separately for the sake of convenience. 

 The lesions produced are not fundamentally different from the other members 

 of the poxvirus group. They have been likened to the cellular lesions of mol- 

 luscum contagiosum. The early studies of fowlpox have been fully reviewed 

 by Goodpasture (1928). Fundamental knowledge concerning the interactions 

 between virus and cell, which ultimately develop the massive lipoidal in- 

 clusion incorporating numbers of virus particles, has advanced little since 

 Ludford's review (1951). The inclusions have not been found in fibroblasts or 

 macrophages in tissue culture, or even in chick epithelial cells in tissue 

 culture. This may be due to the destructive property of the virus for these 

 latter cells when cultured (Bang et al., 1951). Fibroblasts, which normally 

 resist the virus, may be destroyed under special conditions (Kohler and 

 Schreibel, 1956). 



2. Effect on Nucleolus 



The normally homogeneous nucleolus of these chorioallantoic cells appears 

 in the late stages of infection of the chorioallantoic membrane as a structure 

 of coiled or tangled strands (Bang et al., 1951). Although this conversion has 

 also been illustrated by Morgan and Wyckoff (1950), it has not been further 

 studied. It should be reconstructed from serial sections (Bang and Bang, 

 1957) and compared with similar preparations of the known changes in 

 nucleolar structure produced by adenosine and other nucleosides, nucleotides, 

 and benzimidazole (Hughes, 1952a; Lettre and Siebs, 1954). It is possible that 

 an excess of nucleosides is formed during the early phases of virus growth 

 (this is indicated by the Feulgen-positive matrix of the other poxvirus in- 

 clusion), and that this influences nucleolar morphology. A similar change in 

 the shape of the nucleolus has been reported in rabbit fibroma (Bernhard et 

 al, 1955). 



3. Development Sequence 



It is the aim of morphological studies to describe how the various units 

 are formed. It seems to this reviewer that a premature attempt to deduce 

 a sequence of changes from a variety of changes, when the deduction is not 

 based even on a statistical correlation with time, is to obstruct the game. 

 Present knowledge either of the integrity of any phase, or of the mechanisms 

 by which the changes develop, supports no sequential conclusions. When 

 large numbers of cells have been infected with identifiable particles and 

 these in turn have been followed in the electron microscope during the first 

 "cycle" of virus development, then a proposed developmental series may be 



