50 ANIMAL PARASITES. 



the immigration of the greater part of them into the liver, &c, 

 probably takes place very rarely by their first of all piercing 

 through the walls of the stomach, then moving upon the serous 

 coat of the peritoneum in the free abdominal cavity, and only 

 penetrating through this into the parenchyma of the organ 

 selected, for example, the liver. It is true, the latter has not as 

 yet been completely refuted by the fact that Leuckart found no 

 embryos upon the serous coat of the liver. From the size 

 of the surface under examination, he might have overlooked 

 them. 



I also return again to my original supposition, which I after- 

 wards gave up, that the active migration is probably followed by 

 a passive one in the stream of the blood* By the careful inves- 

 tigations of Leuckart, who found free embryos four times in the 

 main branch of the vena porta at its entrance into the liver 

 (whilst he never succeeded in discovering free embryos in the 

 mesenteric and gastric veins, in the lacteals, or on the serous 

 coat of the stomach and liver), I feel convinced that the 

 brood is introduced into the circulation of the blood by the 

 active penetration of the tissues, and then a passive migration 

 with the blood is commenced. In this way the immigration of 

 the brood into the liver would be easily explained, the embryos 

 getting into the vena porta, and remaining fixed in the finest 

 ramifications of the capillary system of the liver ; and into the 

 brain and other parts situated more peripherically, by the 

 embryos making their way into blood-vessels (e. g., the vena cava, 

 the capillaries of the liver, and especially those of the lungs), 

 which conduct the blood, and with it the brood, towards distant 

 regions of the body. Nay, I even go a step further, and per- 

 ceive in the small size of the brood of the last-mentioned Tanice, 

 as compared with the size [of that] of the allied Cestodea in 

 the bodies of cold-blooded animals, an indication of the fact that 

 nature has positively prepared this smallest brood for migration 

 through or into the capillaries, and destined them to this purpose. 

 Lastly, it is perhaps not venturing too far, if, in the pigment on 

 the heads of certain Cysticerci, we should recognise an additional 

 proof of the previous migration of these specimens through the 

 blood of man, or of certain mammalia. 



Moreover, I also subscribe to Leuckart's assumption that, 

 in the choice of its dwelling-place in the body of its new 

 host, the immigrant embryo is not driven to it by a certain 



