4 BULLETIN 1449, U. S. DEPARTMENT OF AGRICULTURE 
duction. Hauser thus considers the changes in the cells more im- 
portant than Ribbert's disturbance of tissue tension. 
Von Hansemann (7) went even further, considering the increased 
growth energy of cells due to a gain in the power of independence 
and coincidental loss of differentiation of chromatin, brought about 
by chromatic changes in the nucleus with unequal division of 
chromosomes. 
THE THEORY OF NERVOUS INFLUENCE 
Disturbance of trophic innervation has been ascribed as the cause 
of neoplasms, the overgrowth of tissue being the result of local irreg- 
ularities of nervous influence, particularly in neoplasms that are of 
nerve origin, formed in the course of nerve trunks. It would be dif- 
ficult to conceive of the formation of all neoplasms as due to such a 
cause. 
THE PARASITIC THEORY 
The parasitic theory accepted in the Middle Ages was revived some 
time ago, especially by investigators of cancer, but this hypothesis 
has received less credence than other theories. Protozoanlike bodies 
with marked degenerative changes of the cells have been found in 
neoplasms by Thomas (20), Russell (18), Gaylord (6), von Podwys- 
sozki (16) , and others. This, however, indicates only their accidental 
presence in a favorable soil produced by the degenerative changes 
and the accompanying chronic irritation, but throws no light on the 
causation of neoplasms; moreover, none of these observers describe 
exactly the same bodies that they consider as protozoan parasites. 
In line with the advocates of the parasitic theory are such inves- 
tigators as Scheurlen (19), Doyen (£), and others, who have found 
bacteria in neoplasms and described the Bacillus neoformaris as the 
cause of cancer. The " Plimmer bodies," which are of blastomycetic 
origin, are said by Plimmer (15) to be the cause of cancer. 
One of the principal objections to the parasitic theory of neoplasms 
is the difficulty of reproducing the disease by transplantation 
of fragments of neoplasms of one animal to an animal of another 
species. 
Late experiments by Jensen (10), Loeb (12), and others in the 
transplantation of tissues have been successfully carried out, but these 
experiments prove only that the cells of malignant neoplasms of an 
animal can be transplanted at times to another animal of the same 
sort and continue to live and grow in the same fashion that a 
metastatic growth forms a secondary neoplasm. They do not prove 
the parasitic nature of neoplasms, because in most of them neither 
protozoa nor bacteria can be found. Benign neoplasms can not be 
transplanted. 
CLASSIFICATION OF NEOPLASMS 
Up to the present time no classification embracing all neoplasms 
has been accepted as satisfactory. Neoplasms may be classified clin- 
ically as to their nature, histogenetically as to their structure, or 
morphologically as to their shape. 
