200 M. M. Mercaur 
solution, or Locxe’s fluid, and be opened under a Zerss binocular 
dissecting microscope (magnification fifty diameters) and rapid 
observation be made of the forms found, all especially interesting 
phenomena bring noted. This preliminary survey is important for 
comparison with later appearances which may be suspected of being 
abnormal. The intestinal wall and contents would then be separated 
from the Opalinae by pushing the former to one side with dissecting 
needles. The Opalinae would then be covered with a thin cover-glass 
and, after a few moments waiting to allow the edges to dry, the 
culture would be sealed with Cheeseborough Manufacturing Company’s 
white vaseline. Wax is not a satisfactory sealing for slide cultures 
which are to be studied with an immersion lens, as pressure upon 
the cover-glass tends to cause leaks in the wax sealing. These 
slide cultures live sometimes as much as two days, but often die 
within eight to twelve hours. Similar slide cultures were often used 
for studying the adult Opalinae, the cover being supported by a 
couple of very fine hairs. The slide cultures of adult Opalinae may 
live three days, though more die the first or second day. 
For studying the finer structure of living Opalina the binucleated 
species are, as already said, by far the better, but not all individuals, 
even of O. intestinalis, are equally clear. Sometimes one finds nuclei 
in which while alive one can observe with remarkable clearness the 
chromosomes, the spindle fibres, and the achromatic granules. It is 
certain that the structures described in the dividing nuclei are not 
artifacts, for they have been observed not only in preserved material, 
but in the living animal as well. Probably no one really gives 
much weight to the sweeping objections that have been made to 
cytological studies as dealing largely with artifacts, yet many reagents 
do undoubtedly produce artifacts which are likely to be misleading; 
it is therefore a satisfaction to be confident that one is describing 
natural structures and not things that have been produced by mani- 
pulation. 
Not only does one find many living individuals which do not 
show their nuclear structure clearly; occasionally one is even unable 
to distinguish the nuclei at all. One must usually carefully observe 
a good many individuals before finding one in which the nuclear 
structures are very clearly seen. It is interesting to note that the 
posterior nucleus is often clearer than the anterior. At first I 
thought this was due to the fact that the protoplasm of the anterior 
end of the body is more dense than that of the rest of the body, 
but there is a further and even more important reason for this 
