Opalina mitotica. 91 
the chromatin reticulum Iying just beneath the nuclear membrane. 
Could the lack of arrangement of this achromatie material in a 
spindle, and the lack of a centrosome, allow the failure promptly to 
complete mitosis? Among plants the phanerogams have no centro- 
somes, but they do have achromatic spindle fibres. The achromatie 
part of the mitotie figure is not wanting in them. In Opalina the 
achromatic material does not assume any specially marked form in 
connection with mitosis. Can it be this lack that allows the nucleus 
to “rest” before completing its mitosis? 
In the mitosis of other genera the achromatice material is pro- 
minent. In the mitosis of Opalina it has a diminished, apparently 
a very slight, role, if any. The chromatin seems to be the active 
element. Mitosis in full sized Opalina intestinalis and O. caudata is 
very slow, occupying from a few hours to several days. Yet in 
these species it becomes complete before the nuclei come to rest. 
The chromatin of the Opalinas seems to be under no compulsion 
from centrosomes or other achromatice source. It is ordinarily very 
deliberate in its self-directed mitosis. In O. mitotica it chooses to 
come to rest before completing the mitosis and there is nothing to 
say it nay. There seems to be some tendency toward suppression 
of divisions of the body and toward promotion of division of nuclei. 
This added relative emphasis upon nuclear division is suffieient in 
O. mitotica to cause the nuclei to enter upon division, but it is not 
enough to carry the division promptly to completion. I see no way 
of finding any mechanical explanation of the failure promtly to 
complete the mitosis, nor do I see any necessity for seeking such a 
mechanical explanation. Might we not come nearer the truth if we 
should say that the chromatin does not choose to complete the 
mitosis? The further question why it does not so choose seems 
beyond our ken. One cannot but doubt if the mitosis would remain 
incomplete, even in one species of this genus, were the mitotie 
figure a fully developed one. It may be the imperfection of the 
achromatic part of the mitotie figure, throwing all the activity upon 
the chromatin, that allows this departure from the otherwise uni- 
versal behavior in mitosis. 
I desire to express to Professor Powers, the discoverer of this 
species, my heartiest thanks for the privilege of desceribing and 
discussing it. 
Oberlin, Ohio, U.S. A., May 15th, 1911. 
