362 BIOLOGY: GENERAL AND MEDICAL 



The spirochetes have actually been seen in the mother's 

 body in the ovaries, and enter the eggs themselves where 

 they appear as congeries of fine granules. 



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. 



FIG. 131. Spirochaete obenneieri. (Novy.) Rat blood. X 1500. 



The Leptospira icteroides now thought to be the micro- 

 parasite of yellow fever, is harbored and transmitted by 

 the mosquito Aedes (Stegomyia) calopus. 



In considering the reciprocal relations of the parasites 

 and their hosts it is important to remember that the host 

 is necessary to the parasite and that his untimely death 

 may interrupt one of the developmental cycles by which 

 the permanence of the parasite is secured. The con- 

 tinuance , of the parasitic relationship may therefore 



