38 AN INTRODUCTION TO THE STUDY OF VIRUSES 
as Shope says, to fill a 200 c.c. flask. The most common sites for the 
warts appear to be on the inner aspect of the thighs, the abdomen and 
on the neck and shoulders. The warts are black or greyish black in 
colour, well keratinized, and the upper surfaces are frequently irregular 
or fissured. The lateral surfaces of the warts appear vertically striated 
because each individual wart is composed of closely-packed and almost 
homogeneous vertical strands of tissue. Sections of warts show a 
white or pinkish white fleshy centre, and the upper portions of its 
lateral surfaces are greyish black and keratinized. Shope found that 
the disease was easily transmissible to healthy rabbits by dropping 
some of a suspension of warts upon a portion of the skin which had 
been shaved and lightly scarified by means of a needle or a piece of 
sterilized sand-paper. 
Infected rabbits exhibited no clinical symptoms and appeared to 
be in normal health. The virus was found to pass Berkefeld filters of 
V, N, and W porosity. . 
Rous (1943) describes how this rabbit papilloma virus is capable of 
infecting the epidermal tumours which can be produced experimentally 
by means of tar and other chemical carcinogens. It is significant that 
some of the tar papillomas undergo an abrupt change into carcinomas 
when the virus lodges in them; for the alterations which take place 
then resemble in all obvious respects those occurring when virus- 
induced papillomas become cancerous. 
Before concluding this discussion of tumour-forming viruses, it 
may be of interest to mention an unusual host for this type of virus, 
Rana pipiens, the leopard frog. Lucké (1938) describes how this frog is 
commonly affected with a carcinoma of the kidney. It is a particularly 
interesting tumour because the cell nuclei frequently contain large 
acidophilic inclusions which are similar to those found in herpes and 
certain other diseases known to be due to viruses. The tumours always 
originate from the renal epithelium and exhibit its morphology; 
they range in aspect from benign adenomas to malignant carcinomas 
and behave accordingly. The causative agent is strictly specific, acting 
only on leopard frogs and the renal epithelium. 
There is a group of interesting viruses which attack only the larval 
forms of Lepidoptera and some Hymenoptera (sawflies) and give rise 
to what are known as Polyhedral diseases. The outstanding character- 
istic of these diseases is the formation within the body of the host of 
enormous numbers of many-sided crystal-like bodies from which 
the name of this type of disease is derived. 
