228 M. M. Mercarr 
nucleolus and their average size may be different in different 
nucleoli. Asa rule the size of the vacuoles is in inverse proportion 
to their number. 
Sections stained with methylen blue often show an interesting 
condition in many nucleoli (Figs. 18 and 19, Pl. XV). The nucleolus 
proper is stained a bluish green. This portion is spherical. In one 
or two regions on its periphery, it bears cap-like structures which 
are stained a clear blue darker than the pale blue of the nucleolus 
proper. This is no accidental condition, for it is present in almost 
all nuclei seen upon these slides which were made from two diffe- 
rent lots of Opalinas. Vacuoles are not seen in the nucleoli which 
show these blue caps, though in other nucleoli upon the same slides 
vacuoles are found. The history of these peculiar nucleoli has not 
been followed, so nothing can be said as to the meaning of the 
conditions found. 
The behaviour of the nucleolus in dividing nuclei is interesting. 
ZELLER observed that in dividing nuclei the nucleolus did not divide 
but remained intact in one of the daughter nuclei, the nucleolus of 
the other daughter nucleus being a new structure. I can fully con- 
firm this for O. intestinalis and O. caudata. The nucleolus is less 
easy to see in the smaller nuclei of the multinucleated species, and 
as my material of the multinucleated forms shows comparatively 
few nuclei in division I have not taken the considerable time re- 
quired to study the nucleoli carefully in them. 
To Zruuer’s interesting observation I would add the further 
facts: — first, that in O. intestinalis the old nucleolus remains in 
the posterior of the two daughter nuclei (Figs. 70—72, Pl. XIX), 
and second, that in O. caudata this relation is usually reversed, the 
old nucleolus remaining generally in the anterior daughter nucleus 
(Fig. 82, Pl. XX). In the many hundreds of nuclei of O. intestinalis 
examined I have found but a single exception to this rule (Fig. 74, 
Pl. XIX). In this young daughter cell whose nucleus is still in the 
dumbbell stage of division, the nucleolus was found in the anterior 
part of the nucleus near the constriction. The narrow tube connec- 
ting the daughter nuclei was not too small to allow the nucleolus 
to pass through it and reach its usual position in the posterior 
daughter nucleus, but that the nucleolus would have done so does 
not seem very probable. 
In a large majority of cases, in dividing 0. caudata the old 
nucleolus remains with the anterior daughter nucleus, yet one occassio- 
nally finds these relations reversed. 
