146 MICROBES AND TOXINS 



that the disease was hereditary, but in reality it is a case of 

 contagion or of transmission at short radius ; the placental 

 filter has been injured (Chamberland's experiment). Nowadays 

 heredity in tuberculosis is no longer believed in; what is 

 inherited is at most a physiological predisposition of the soil 

 (and even that is a vague and uncertain idea), or, alternatively, 

 conditions of life in which the bacillus, everywhere to be 

 found, can flourish. There is no hereditary infection in the 

 strict sense unless the fertilised ovum is infected by the 



Fig. 57. The spirochsete of Schaudinn in the liver of a congenital 

 syphilitic. (From a microphotograph. Magnified 3,000 times. ) 



parasite (among the vertebrates either from infection of the 

 female cell or of the spermatozoon, or of both) ; in such a 

 case the disease is truly congenital. There is no certain 

 example of such a fact among the bacterial diseases, and this 

 is the reason why the idea of heredity among them has lost so 

 much ground. 



From their power of penetrating cells the protozoa fre- 

 quently infect parasitically the ovum, thus producing hereditary 

 infections. The first thoroughly demonstrated example of 

 hereditary infection was that of the pttrine of the silk-worm, 



