FEBRUARY 2, 1912] 
cancer are now generally agreed, I believe, 
that there are many phenomena connected 
with the development of cancer for which 
this hypothesis of Cohnheim offers a 
wholly inadequate explanation. Moreover, 
what induces these dormant cells to de- 
velop was never determined. A very favor- 
ite theory with cancer specialists has been 
that the cancer cell itself is the only para- 
site, and that no infections could be ob- 
tained on animals unless the living cancer 
cell were present. This hypothesis must 
now be abandoned owing to the discovery 
by Peyton Rous (1911) that sarcoma of 
chickens may be produced in the absence 
of cancer cells, 7. ¢., by cancerous fluid fil- 
tered free from all traces of living cancer 
cells. So far as I know he has not expressed 
any opinion as to the nature of the infec- 
tion which has been separated from his 
ground chicken sarcomata by centrifuging 
and also by filtration through Berkefeld 
bougies, but in the light of the evidence we 
have secured from plants I believe you 
will agree with me that it can be nothing 
else than a living microorganism, minute 
enough to pass through the walls of the 
rather coarse filter. 
In crown-galls I have not found the seec- 
ond method of formation of secondary 
tumors, namely, by the detachment of 
small fragments of the primary tumor to 
be carried in a stream and lodged at a dis- 
tance. This method we should hardly ex- 
pect to find in plants, owing to the fact that 
there is no rapid blood stream such as we 
find in animals, neither does it seem to be 
more than an epiphenomenon in tumor 
growth, the essential thing being the ab- 
normal internal stimulus to cell division. 
The first method of propagation, namely, 
by strands, occurs, however, and parallels 
to my mind very strictly what occurs in 
malignant animal tumors, é. g., in car- 
cinoma, sarcoma, ete. 
SCIENCE 
165 
The existence of the tumor strand in 
crown-gall was overlooked for a long time. 
But last spring in making some sections of 
Paris daisy plants which had been inocu- 
lated with the crown-gall organism and 
bore both primary and secondary galls, I 
saw on cross-section a tumor-strand in the 
inner wood next to. the pith between the 
secondary and the primary tumor and near 
the latter. This was about a millimeter in 
diameter and of a different color, 7. e¢., 
greenish, and easily observed by any one. 
Often, however, this strand is composed of 
a few cells only and difficult to find, even 
with the compound microscope. This, to- 
gether with preoccupation on other phases 
of the research, must serve to explain why 
it was overlooked for so long a time. As 
soon as I saw this parenchyma out of place 
I said, ‘‘ Here is a tumor strand!’’ and be- 
gan to examine many other plants to see if 
it was at all constant—finding it visible to 
the naked eye near the primary tumor in 
perhaps 20 per cent. of the plants ex- 
amined. The question then arose whether 
it was merely local, or could be traced for 
some distance and was of constant occur- 
rence in the normal tissue between the’ 
primary and the secondary tumors. Since 
then many inoculated plants have been ex- 
amined microscopically, and in all of them 
J have been able to find this tumor strand, 
although, as already stated, in many cases 
it is composed of a very few cells. In the 
Paris daisy it usually bores its way be- 
tween pith and wood, or at the inner edge 
of the wood wedge in the protoxylem, ap- 
parently along lines of Jeast resistance. 
(Lantern slides were exhibited, showing 
eross and longitudinal sections of such 
strands from the inoculated plants.) 
On this strand are developed secondary 
tumors, apparently either where the food 
supply is most abundant or where the 
pressure of surrounding tissues is least, 
