8 AN INTRODUCTION TO THE STUDY OF VIRUSES 
Grafting or transplanting is also occasionally used in the study of virus 
tumours of fowls and ducks, and tumours have been successfully 
transferred in this way. 
Transmission by Inheritance 
Hereditary transmission of viruses is not common but there are 
some authentic cases. Placental transmission to the embryo may 
occur in certain diseases such as variola, varicella, Rift Valley fever, 
and rinderpest. Perhaps the most interesting case of hereditary 
transmission of a virus is that of breast cancer in mice. Some strains 
of mice may have an incidence of breast cancer in breeding females 
of 80 to 90 per cent, whilst in others the incidence is almost nil. In 
hybrids between two such strains it has been found that the incidence 
of breast cancer in the offspring depends not upon the ordinary laws 
of inheritance but wholly upon whether the mother came from a 
high or low cancer stock. In other words the cancer of the breast 
in specially inbred strains of mice is caused by a virus found in the 
female mouse’s milk. Until very recently it had been assumed that 
the virus contained in the mother’s milk passed into the body of the 
suckling by way of the alimentary canal. Now, however, experiments 
suggest that infection takes place through the nose and throat and 
this is borne out by the inability to detect the virus in the ingested 
milk in the suckling’s stomach. 
There is a good deal of evidence which suggests that the polyhedral 
viruses attacking caterpillars and other insect larvae may occasionally 
be transmitted through the egg. In an outbreak of polyhedral disease » 
in a stock of caterpillars, there are usually individual larvae which 
do not succumb to the disease. These larvae complete their meta- 
morphosis but seem to carry the virus with them through the develop- 
mental cycle, so that a proportion of the caterpillars of the next 
generation become infected. 
The number of plant viruses which are transmitted through the 
seed is small and the reasons for the rarity of seed transmission are.not 
known though various conjectures have been put forward, the most 
likely being the apparent anatomical isolation of the embryo. There 
are, however, a few authentic cases of the seed transmission of viruses 
such as those of bean and lettuce mosaic. In this context it should 
be understood that all plant viruses which are systemic in their 
hosts are transmitted through vegetative organs such as tubers, 
rhizomes, cuttings, etc. It is because of this that the virus diseases of 
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