32 BIOLOGY: GENERAL AND MEDICAL 



The spirochaetes have actually been seen in the mothers 

 body in the ovaries and in the eggs themselves. 



These observations make clear the transmission of 

 Texas fever by the Ripicephalus bovis. The female 

 tick distended with the infectious blood drops off, ovipos- 

 its, and dies, but the immature ticks transmit the disease 

 to cattle so soon as they reach them. The explanation 

 is found in the passage of the parasites into the egg and 

 the infection of the embryo, though this has not yet 

 been seen. 



Fia. 127. Spirochsete obermeieri. (Novy.) Rat blood. X 1500. 



Even in cases in which the specific parasite has eluded 

 discovery the application of these parasitological prin- 

 ciples has borne fruit. Thus no microparasite has yet 

 been discovered for yellow fever, yet it has been deter- 

 mined that whatever it is, it is harbored and transmitted 

 by the mosquito Stegomyia calopus, and that it under- 

 goes some change in the body of that insect, the perfec- 

 tion of which requires about twelve days. 



In considering the reciprocal relations of the parasites 



