54 Manual of Veterinary Microbiology. 



mother and the tendency of the various germs living 

 in the blood to determine vascular and other lesions. 



Nevertheless, this interpretation, which certain ob- 

 servations sufficiently justify, does not explain all the 

 facts of heredity, and it seems that a simpler mechanism 

 intervenes in some cases. 



We know that the contagious affections of the geni- 

 tal apparatus of the female are readily transmissible, 

 and they will actually be transmitted to the foetus un- 

 less their existence renders gestation impossible, or 

 provokes abortion. 



The disease transmitted to the foetus may cause the 

 death of the latter and its premature expulsion, it may 

 disappear, or, finally, may remain in a stationary con- 

 dition, permitting the complete development of the 

 young animal, in which, at a later period, it may un- 

 dergo fresh evolution. 



Hereditary transmission may be limited to the com- 

 munication of immunity by the diffusion through the 

 placenta of soluble vaccine substances elaborated 

 within the body of the mother. But immunity in the 

 young being may also be consecutive to recovery from 

 the disease with which it has itself been affected. 



■ Indirect or mediate contagion. — We call the contagion 

 indirect when the virulent germs which come from 

 diseased subjects are transported on to healthy indi- 

 viduals after having been deposited on its surround- 

 ings. We have already seen in considering the dis- 

 tribution of pathogenic germs in external media, that 

 they may be encountered in the air, water, the soil, 

 foods, the walls of houses, on mangers, racks, manure, 

 litter, and the various objects and utensils which have 

 been more or less directly in contact with the diseased. 



