FATE OF ILEMATOZOA. 51 



the worms is to be explained by the state of the host as regards 

 digestion and muscular exertion, as well as on the motion of the 

 lymph due to these. 1 K. L.] 



If these Hrematozoa arrived at complete maturity in their host, 

 one would expect to find, not merely a vast and increasing number of 

 adults, but also all the intermediate stages. But no one has hitherto 

 observed anything of the kind ; the Hsematozoa remain for months, and 

 even years (Gruby and Delafond), in the same developmental stage, and 

 without altering in size. Even in cases where the adult worms exhibit 

 some variation in their stages of development, as Lewis observed in 

 certain parasites of the dog, there is a considerable gap between the 

 youngest of these and the Hsematozoa in the blood. These facts 

 point to the conclusion that the intermediate stage between the 

 Haematozoon and the fully developed parasite is passed outside the 

 body of the host. 



The analogy of Trichina also lends support to this opinion. The 

 young of this Nematode are produced viviparously, and like the 

 embryos of the above-mentioned Filaria, wander about in the body of 

 their host, 2 the only difference and that an important one being 

 that they abandon the blood-vessels and betake themselves to the 

 intermuscular connective tissue. In both instances we have a wander- 

 ing from one part of the body to another, though it differs in kind 

 in the two forms. But in Trichina also the result of this wandering 

 is by no means the direct degeneration into the parasitic condition of 

 the adult ; the embryos, on the contrary, remain within the muscles, 

 and, after developing up to a certain point, become encysted, and 

 remain in this condition (as muscle- Trichinae, Fig. 15) until they are 

 swallowed by a new host, when they recommence their wanderings. 



In Trichina, therefore, and in these Hsematozoa, a change from 

 one host to another is necessary before sexual maturity can be reached. 

 From the observations of Ecker, that the Hsematozoa of the rook 

 encyst themselves in the mesentery of their host, one would be 

 inclined to believe that the life-history of Filaria attenuata is to be 

 regarded exactly in the same light as that of Trichina, and that the 

 transference into a new host is brought about by the encysted form. 

 I myself, however, believe that this is really not the case, and that the 

 encysted worms have nothing to do with the developmental cycle of 

 Filaria attenuata, not merely because in this event they ought to be 

 far more abundant than they actually are, but because the contents of 

 these cysts, in the instances that I personally examined, agreed entirely 



1 Scheube, "Die Filarien-Krankheit," in Volkmann's " Sammlung Klinischer 

 Vortrage," No. 232, Leipzig, 1883. 



3 Leuckart, " Untersuchungen liber Trichina spiralis," Leipzig, 1860. 2d ed., 1865. 



