210 PARASITISM 



disease-causing germ, which apparently has no power of migrating 

 from cell to cell (Prowazek, '05). After a number of such divisions 

 the infected cells undergo hyperplasia and hypertrophy; the pressure 

 and possibly the toxins from the organism cause neighboring cells to 

 proliferate until large abnormal growths result. The parasites, in 

 the meanwhile, having exhausted the nutriment of the host cells, 

 form permanent spores, the spore formation being preceded by endo- 

 gamous fertilization processes, as described on p. 147. These spores 

 are stored up in the plant cells until the latter decompose and disin- 

 tegrate in the soil. 



In club root, therefore, we find an analogy not in the form or type 

 of the tumor produced, but in the renewed division energy of tissue 

 cells through the presence of an intracellular parasite. Here, again, 

 infectivity is entirely independent of growth energy of the tissue cell, 

 and dependent upon the parasite alone. The vegetable cell cannot 

 long withstand the inroads of the relatively large parasites, and ulti- 

 mately dies because of them. It is conceivable that a cancer parasite 

 may exist within a cancer cell and serve as a source of continued 

 stimulus to the division energy without causing more harm to the cell 

 than anaplasia or hyperplasia. Such an aspect of the cancer problem 

 was stated as follows in an earlier publication: "It is certainly con- 

 ceivable that a parasite of cancer may be too minute to be seen with the 

 technique at our disposal. At the present time we know a great deal 

 about the yellow fever organism; we know the period of incubation 

 it requires in the human blood ; we know that it requires from twelve 

 to fourteen days to develop in the body of the mosquito before the 

 latter is able to transmit the disease; we know that the disease (apart 

 from blood inoculation) cannot be transmitted in any other way, and 

 yet, knowing all these things, the organism of yellow fever has never 

 been seen. It will pass through the finest filters, and belongs, there- 

 fore, to a group which, until they are actually seen, we must perforce 

 consider as ultramicroscopic organisms. Such parasites might be 

 adapted to life within the epithelial cell as well as the organisms of 

 club root are, and there in the protoplasm might easily be overlooked. 

 It has been suggested that a species of spirocheta is responsible for 

 yellow fever, and spirochetes have actually been found in the kidney 

 of yellow fever victims. But they apparently do not exist as such in 

 the blood or in the mosquito. We know nothing about the life history 

 of the spirochetes as a group; if it is analogous to the life history of 

 most protozoa, we might well look for stages in which the organism 

 is of ultramicroscopic size."' 



Many so-called parasites from human tumors have been described. 

 Protozoa representing all groups of these unicellular animals have 



' Calkins, The So-called Rliythins of Growth-energy in Mouse Cancer, Jour, of Exper. Med,, 

 1908, vol. X, No. 3, p. 304. 



