HEREDITY AND DISEASE 100 



poisons, the gerin-cclls were unable to transmit sucli 

 ininninity. Apparently they were unatVected, even 

 by the presence of these poisons in the blood that 

 passed through the reproductive glands. 



Let us now revert to the typical question of the 

 inheritance of tuberculosis, and inquire as to th(3 

 occurrence of anything that may properly bo termed 

 a fact of heredity. 



It is conceivable, as we have already observed, 

 that the male or female gametes of any tuberculous 

 parent might be infected with tubercle bacilli, and 

 the changes they wrought in such gametes w(ndd 

 doubtless affect the " segmentation nucleus " (the 

 nucleus of the first cell of the new organism); but 

 even this state of affairs is scarcely cognate to the 

 problem of heredity proper, though it nuist later 

 be considered. 



In point of fact, however, so-called hereditary or 

 congenital tuberculosis is practically non-existent; 

 one or two cases have been recorded, but they are 

 merely pathological cariosities. The germ-cells of 

 tuberculous parents are not — save perhaps once in 

 millions of cases — infected by the tubercle bacillus. 

 Furthermore, it has been discovered that the pla- 

 centa, or after-birth, the organ of communication 

 between the mammalian mother and her unl)orn 

 child, possesses the power of filtering the blood ^ 

 which passes through it, so that bacteria whicli 

 may be present in the maternal blood do not, save 

 in a quite infinitesimal proportion of cases, gain en- 

 trance to the body of the child. Thus it may be 



^ This is loosely expressed, but with sunicicnl jiccurarv for fho 

 present purpose. 



