74 



gators tlie liver has been found to be filled with uuiltitudes of them. 

 One investigator has found them iu the minute branchlets of the portal 

 vein, which conducts the blood from the stomach and intestines to the 

 liver. Now, in order to appear in the liver and in the portal vein in 

 such numbers the embryo must make its way into the little branches or 

 capillaries which collect the blood at the stomach, aud then be washed 

 by the blood current into the liver. When they become lodged iu the 

 liver they again migrate actively and tunnel throngh the mass of the 

 liver in all directions. The little channels are made slowly. They begin 

 as a minute point, aud become gradually larger as the parasite increases 

 in size and changes its j^osition. About the ninth day after the embryos 

 have been swallowed some of the parasites may be as large as a flax- 

 seed; most of them will be smaller. They will then be little oval water- 

 bags, with a whitish thickening at the end. 



Some of the parasites seem to pass the entire length of the intestine 

 before they j^enetrate the walls ; but when they reach the rectum they 

 pass through, and, becoming lodged between the layers of serous mem- 

 brane, develop there. This probably accouuts for the presence of the 

 considerable numbers which are found in the pelvis. 



The destination at which these parasites arrive has its influence on 

 their development. When they have migrated to a point between 

 serous tissues which may be easily spread apart, and ofter little resist- 

 ance to the growing parasite, a certain proportion of them seem to 

 thrive aud arrive at a stage in which they may continue development 

 by the proper exchange of hosts. If this exchange is not effected the 

 jiarasite may remain unharmed for a long time while awaiting this 

 opportunity. Should they die from any cause a slow change in the 

 appearance of the parasite is noticed. The fluid it contains becomes 

 milky and limy. The serous sac surrounding it becomes thicker; finally 

 such changes have occurred that iu place of a soft sac a round, hard 

 sphere of lime covered by a thick membrane may be found. The re- 

 mains of the parasite may still be seen by careful dissection between 

 the cover and the limy deposit. 



But when, on tlie other liaud, they arrive at the liver they seem to 

 have reached a place of destruction; for if they do not kill the invaded 

 host within two weeks, a period long before they could continue their 

 life in another host, the same degenerative processes seem to atfect them, 

 in their earlier stages, which destroy the older individuals in other or- 

 gans of the body. This seems in i^art due to the i)ressure which the liver 

 cells and capsnle exert upon them. These statements seem substanti- 

 ated by the presence of either scars or calcareous nodules in the liver 

 and of no large cysticerci, except where the loose serous membrane may 

 have permitted their growth. 



These parasites have occasionally been observed in other parts of the 

 system than those mentioned, as iu the lungs, heart, aud muscles. 



Disease. — The harm that Tcvnia marpinata does in its young stage 

 depends much on the degree of infection. In experiments animals have 



