476 
MONTGOMERY. 
[Vol. XV. 
tially similar to those of Linens ; in the metanemerteans they 
are too small for satisfactory study.) 
These nuclei are very variable in shape, all extremes being 
found between ovoid or oval and elongate rod-like forms. But 
they are rarely angular. I have remarked in a previous con¬ 
tribution that the nuclei of the muscle cells are more variable 
in form than those of the cells of any other tissue in the 
nemerteans, and now I would offer the following explanation 
for this variability : when the muscle fiber (a single, smooth 
fiber with attached nucleus constitutes a muscle cell) contracts, 
this contraction must produce likewise a contraction (shorten¬ 
ing) of the nucleus ; but when the fiber expands the form 
of the nucleus must become more elongate, corresponding to 
the elastic extension of the fiber, for the fiber cannot contract 
without causing a shortening of its nucleus, since the latter is 
closely adherent to it. 
One very small nucleolus is usually to be seen in each 
nucleus (Figs. 51-54, 56); sometimes it does not appear to be 
present (Fig. 55), but whether in these cases it is absent or only 
escapes observation by reason of its minute size, it is difficult 
to decide; in the greater number of nuclei it may be seen by 
careful focussing of the microscope. It most usually lies very 
close to the center of the mass of nucleoplasm, so that if the 
nucleus be larger at one pole than at the other it is situated in 
the larger end, while in elongate nuclei, of nearly equal diam¬ 
eter throughout, it usually lies at an early equal distance from 
both ends of the nucleus. The nucleolus may be said, as a 
general rule, to occupy the center of the nuclear substance, 
and is not often markedly excentric; in none of the other cells 
examined in the course of these investigations did the nucleoli 
show a similar tendency to occupy the center of the nucleus. 
The nucleolus always stains differently from the chromatin. 
The relative amount of chromatin varies in different nuclei. 
It is always found, after the action of various fixatives, to occur 
in the form of small granules, which are connected by delicate 
irregular fibers, which stain exactly as the granules do. The 
nuclear sap stains faintly with haematoxylin (this has not 
been shown in the figures). The nucleolus is either in contact 
