Opalina. 225 
constricted in the middle at the last division and drawn out to a 
thread (Pl. XVI, Figs. 34, 35, 37). This thread persists for a long 
time, disappearing, generally, as the two nuclei are entering upon 
the next division. Frequently the connecting thread is much bent 
or even coiled, being far longer than the shortest line between the 
two nuclei (Figs. 35 and 37), a condition which suggests that the 
thread elongates by its own growth. 
The two nuclei divide at the same time, becoming first spindle- 
shaped, then dumbbell-shaped, and finally separating into two 
daughter nuclei which are still for a long time united by the thread 
which indicates their common origin. While the division of the two 
nuclei is occurring, the body divides (P]. XVII). This is usually during 
the anaphases, but one often finds the body still but partially divided 
when the nuclei are entering on the telophases. One often sees a 
daughter cell with only a single nucleus, but this, if normal, is always 
in an anaphase or early telophase stage of division (Figs. 32, Pl. XVI; 
43, Pl. XVII; 54, Pl. XVIII). It is during the early telophase that the 
nucleus constricts into two (Figs. 32, Pl. XVI; 60—65, Pl. XIX). 
The nuclear membrane is very definite and clear, not thick, 
but very firm and strong. This is indicated by the fact that the 
connecting strand between the daughter nuclei persists for so long 
a time. It is seen even more clearly when the living animals are 
crushed by pressure upon the cover-glass, causing the nuclei to come 
out into the surrounding salt solution. Such isolated nuclei, even 
when connected by very slender threads, one seldom succeeds in 
causing to break apart by the most violent currents one can produce 
by pressing upon the cover-glass. Often one of the two united 
nuclei will be held in place by its connection with the broken body, 
while the other nucleus projects into the clear liquid. One can then 
make it jerk about and tug violently upon the thread that holds it, 
yet without breaking the thread. I kept one such pair of nuclei 
for three days, trying several times daily to break the thread by 
violent currents, but even the third day it held as firmly as ever. 
It is very evident that under this severe treatment the thread con- 
necting the nuclei does not stretch. It seems not to be at all elastic. 
Contre & Vaney describe the endosare spherules as arising from 
the nucleus, from which they emerge through an opening in 
the membrane. We will return again to this point. It is well 
here merely to emphasize the remarkable toughness of the nuclear 
membrane, which could be penetrated only with the greatest difficulty, 
unless it were weakened (chemically?) at some point. 
