THE UNIQUENESS OF THE INDIVIDUAL 



ably, the future development of the immunological defences of 

 the child. Nearly thirty years ago Traub showed that a virus 

 disease of mice, lymphocytic choriomeningitis, can be trans- 

 mitted from a mother to her unborn young; the young were 

 accordingly unable to develop resistance to the virus in later 

 life, and might transmit it to their young in turn. In this par- 

 ticular strain of mice, virus and host had come to a live-and-let- 

 live arrangement by which neither killed the other, and reflec- 

 tion will show that, had this not been the case, the phenomenon 

 could hardly have been discovered. But here is an example of a 

 ''hereditary'' infectious disease, running in a family, but trans- 

 mitted from mother to young because each generation not 

 merely infects its successor but abolishes its successor"'s power 

 to rid itself of the disease. ''Genetic predisposition'' is therefore 

 not the only possible explanation of a tendency of certain 

 diseases to run in families. 



Under normal circumstances, the mere incorporation by a 

 foetus of some of its mother"'s cells need not be expected to 

 lead to evil consequences; it does not normally happen because, 

 as I explained above, a frontier which lets through cells would 

 let through undesirable immigrants as well. One can be con- 

 fident that maternal cells are not admitted into the foetus, 

 because if they were, the young should acquire a complete or 

 partial tolerance of homografts transplanted from the mother, 

 and this happens very rarely, if at all. Cancerous cells, however, 

 are distinguished by their invasive properties, and there are 

 half a dozen cases on record of the apparent transmission of 

 malignant melanomatosis from a pregnant mother to her child. 

 There can be little doubt that the melanoma cells actually 

 crossed the placental frontier and established themselves in 

 the foetus. An adult human being certainly, and even I think 

 a newborn, will destroy homografts of malignant cells. It would 

 not inevitably be disastrous for a foetus to allow, because it 

 could not prevent, the growth of a foreign malignant tumour, 



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