302 THE ANIMAL PARASITES OF MAN 



evaginates, so that the receptaculum capitis now forms the inner 

 surface of the hollow head, which eventually becomes solid. 



Our knowledge of the development of cysticerci in the wide 

 sense of the word is limited almost exclusively to that of a few true 

 " bladder worms" (cysticerci); in other cases we know either only 

 the terminal stage, i.e., the complete larva, or, exceptionally, one of 

 the intermediate stages, but we are not acquainted with a complete 

 series ; the description must therefore be incomplete. 



We know from feeding experiments that, after the introduction 

 of mature proglottids or of the fully developed ova of Tcenia 

 crassicollis (of the cat) into the stomach of mice, the oncospheres 

 escape from the shell in the middle portion of the small intestine, 

 and a few hours later penetrate into the intestinal wall by means 

 of a boring movement ; they have been found in this position twenty- 

 seven to thirty hours after the infection. By means of this migration, 

 for whicli purpose they employ their spines, they attain the blood- 

 vessels of the intestine; indeed, already nine hours after the infection 

 and later they are found in the blood of the portal vein, and in the 

 course of the second day after infection they are found in the 

 capillaries of the liver, which these larvae do not leave. 



Leuckart, in experimental feeding of rabbits with oncospheres 

 of Tcvnia serrata (of the dog), found free oncospheres in the stomach 

 of the experimental animal, but not in the intestine : however, 

 he came across them again in the blood of the portal vein. The 

 passage through the blood-vessels to the liver is the normal one 

 for those species of Tccnia the eggs of which become larvae in 

 mammals ; even in those cases in which the oncospheres develop 

 further in the omentum or in the abdominal cavity (Cysticercus 

 tenuicollis, C. pisiformis), there are distinct changes observable in 

 the liver that lead one to the conclusion that there has been a 

 secondary migration out of the liver into the abdominal cavity. 

 Indeed, one must not imagine that the young stages of the Cestodes 

 are absolutely passive ; once they have invaded an organ they travel 

 actively, and leave distinct traces of their passage. 



In other cases the oncospheres leave the liver with the circulation, 

 and are thus distributed further in the body; they may settle and 

 develop in one or more organs or tissues. Many oncospheres may, 

 by travelling through the intestinal wall, penetrate through it and 

 attain the abdominal cavity direct ; some, perhaps, pass also into the 

 lymph stream. Where there are no blood and lymphatic vessels in 

 the intestinal wall, as in insects, the oncospheres attain the body 

 cavity or its organs direct ; in short, they never remain in the 

 intestinal lumen itself, and only rarely as in Hymeuolepis miirina 

 of the rat do they remain in the intestinal wall. 



