Every layman knows that tuberculosis tends to cling to families. "The consumptive," says Hippocrates, "is born of a consumptive," and the search for an explanation of this fact has long engaged general interest. The earliest and most obvious explanation assumed an hereditary transmission, but the notion of an inherited tuberculosis, in a mendelian sense, could not, of course, survive the discovery of the cause of the disease. Other notions which have enjoyed more or less vogue at various times are, for example (a), germinal transmission by ovum or spermatozoon, such as is known to occur in silkworm pebrine. A certain amount of experimental evidence (which will be discussed in some detail later), supports this view, but the theory is open to criticism on various grounds, among others, that tuberculous infection of either germ cell would probably render it incapable of function, or, at least, lead to the early death of