Sept. x6,1918 Tisstce Invasion by Plasmodiophora brassicae 
555 
such a stage has been found. Here, again, the plasmolysis of the host 
cells makes the parasite stand out more clearly than it otherwise would. 
Figure H shows a plasmodium passing through another cell wall. The 
section in which this plasmodium was found had been left overnight in 
acid alcohol in order to remove thoroughly the stain from the cell walls. 
While this was being accomplished, the stain was also removed from the 
plasmodium to such an extent that its nuclei can no longer be distin¬ 
guished. Figure I shows a rather large plasmodium in the process of 
passing through the end of a cell in the region of the cambium. 
Late stages in the process of cell-wall penetration have not been found. 
It may be that these stages require a comparatively short period of time. 
It may also be that a part of the difficulty lies in distinguishing them 
from the early stages of penetration because in some cases it is not easy 
to determine in which direction the plasmodium is passing. The general 
direction in which migration is taking place helps to determine this point. 
The bending of the cell wall, where this occurs, is further evidence of the 
direction of passage. In this way we know that the plasmodium shown in 
figure I is passing from the cell on the left to the cell on the right. In a 
number of cases it was observed that the nucleus of the old host cell 
lies near the plasmodium as it passes out into the new host cell. Figures 
D, G, and H show such cases. Figure J shows an instance in which the 
plasmolysis of the host cells seems to have broken a migrating plasmo¬ 
dium into two parts. No hole could be seen in the cell wall, however. 
A careful study of tissues through which plasmodia are rapidly migrating 
has been made, but in no case has a hole been found in a cell wall except 
when a plasmodium is actually passing through the wall. This would 
seem to indicate that the holes close up soon after the plasmodia pass 
through. The drawings show that the hole varies considerably in size 
in the different cases studied. 
The migrating plasmodia undoubtedly grow and divide as they pass 
from cell to cell, but this part of their behavior has not been sufficiently 
studied up to the present time. In all cases observed the cell out of 
which the plasmodium is passing is being left free of infection. Such 
cells act as temporary hosts for the plasmodia that pass through them 
without leaving any trace of the cells having been infected. While some 
of the plasmodia move deeper and deeper into the tissues, others remain 
permanently in certain of the host cells. Here they grow and in time 
mature spores. The migrating plasmodia are in all cases small. They 
contain few nuclei and are relatively free of the oil that is so abundant 
in the larger plasmodia. It is not known what determines whether a 
plasmodium shall stop in a given cell or shall pass on to some other cell. 
The writer has observed, however, that the infecting plasmodia in the 
edges of diseased areas tend to keep a certain distance apart as though 
they might have a repelling influence on each other. It is not believed 
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