STERILITY 123 



countless number of villi rapidly sprout out from the 

 allantois, the embryo will die from starvation during 

 the eighth week, and in a few days be discharged. It 

 may, therefore, be taken for granted that there is a cer- 

 tain amount of danger at the end of the third and sixth 

 weeks, but that the most critical period is about the end 

 of the seventh or beginning of the eighth week ; for unless 

 the villi appear in time, and succeed in coming into suffi- 

 ciently intimate relation with the uterine vessels, the 

 developmental process is of necessity forever arrested." 



Abortion among sheep seems to be largely due to debili- 

 tating conditions due to insufficient and unsuitable food, 

 although Heape 1 has pointed out that shearling ewes 

 are more apt to abort than those of maturer age. 



118. Contagious abortion and sterility. The most 

 insidious, widespread and generally important cause of 

 sterility, especially among cows, is due to a germ infection 

 (Boot, abortus) which is recognized under the terms con- 

 tagious, or infectious abortion. It is found oftener in 

 herds of dairy cattle than among beef breeds, not because 

 dairy animals are more susceptible to this disease, but 

 because they are generally handled in such a manner as 

 to provide more favorable conditions for its spread. 

 Beef breeds are generally less closely housed and they 

 are more frequently permitted to calve on the pastures, 

 thus avoiding two common circumstances favorable for 

 the transmission of the disease. The infection is carried 

 chiefly by the bull. If a healthy bull is permitted to 

 serve a cow infected with the germ of abortion, he will 

 generally transfer the infection to all cows which later 

 may be served by him. The disease is not communicated 

 to any important extent from cow to cow by merely 



1 Heape, "Abortion, Barrenness and Fertility in Sheep." 



