194 THE PARASITISM OF 



we can judge, infection by the alimentary canal is probably the 

 only method of infection. 



Conclusions as to Protozoa in Rabbits. 



It may be taken as established in the case of the Coccidium 

 oviforme attacking the rabbit that : 



1. A most important portion of the developmental cycle of 

 this parasite takes place only external to the body, under aerobic 

 conditions. 



2. Influences occurring outside the body delay, and even pre- 

 vent the external sporulation of the parasite, thus interfering with 

 its infective power. 



3. The host cannot be infected by coccidia inoculated directly 

 from animals already suffering, thus proving that the disease, 

 though infectious, is so only in a very special way. 



4. The parasite infects the host by passing into the aUmentary 

 canal, where it meets suitable conditions for its future development. 



5. The parasite enters and grows within epithelial cells without 

 necessarily destroying them, and causes great proliferation of the 

 neighbouring epithelium. 



The Question of Parasites in Cancer. 



Having drawn attention to the development of this well-recog- 

 nised organism^ and its effect on the tissues, I wish now to consider 

 certain appearances which may be recognised in carcinomata, and 

 to inquire what evidence there is of their parasitic character. 



It is now more than forty years since Virchow* drew attention 

 to the structure of certain cells occurring in tumours which 

 appeared to contain inclusions of extraneous origin. At that 

 time considerable discussion arose concerning these cell inclusions. 

 The chief opinions then advanced in explanation of the bodies 

 under observation were that they arose from the endogenous for- 

 mation of cells, along with alteration of their nuclei, so as to give 

 the appearance of homogeneous spheres (Virchow), or that they 

 were due to the imbibition of albuminous or watery fluids, probably 

 a part of a degenerative change (Henle, Bruch, and others). 



*Virchow, Arch. f. path. Anatomie, etc., Bd. i, 1847, Taf. ii, Fig. 5, /^ and /, 



and Band iii. 



