Opalina. 257 
membrane. The spindle usually does not fill the whole nucleus, the 
nuclear membrane apparently growing more rapidly than the rest 
of the nuclear structures. During the whole mitotie cycle the nuclei 
are in constant growth, as is indicated by their increase in size. 
That the nuclear membrane shares in this active growth is shown 
not only by the fact just mentioned that the spindle does not fill 
the nucleus, but also by the fact that in the late telophases the 
thread connecting the two daughter nuclei grows even to unnecessary 
length and becomes coiled. 
The usual peculiar form of the spindle, with acuminate ends, 
is instructive. It does not seem as if the chromatin fibres uniting 
the chromosomes to the poles of the nucleus can be contracting, 
for they are much, and quite irregularly, bent. They are not 
taut, as if pulling upon the chromosomes. Yet it is barely possible 
that the minute transverse fibrils connecting the longitudinal 
fibres of the spindle are drawing these together with sufficient force 
to bend them into the irregular bows which are seen. Naturally, 
at the narrowed ends of the nucleus, these transverse fibrils are 
more numerous in a given area than they are near the equator, 
where the nucleus has nearly its original diameter. The first 
impressions one receives from such a nucleus as that shown in 
Figs. 49—52, Pl. XVIII, may be that the spindle is elongating and 
pushing the nuclear membrane in front of it; yet the whole character 
of the chromatin fibres of the spindle is such as to suggest that 
they are pliable and not so stiff as any such hypothesis of the force 
of their elongation would imply. The fibres are usually quite irre- 
gular and curved, and it seems impossible to think of their pushing 
with any appreciable force. Such irregular fibres may exert some 
pull, but that they can effectirely push is unbelievable. ’) 
The migration of the chromosomes is accompanied by a perceptable 
thickening of the chromatin fibres connecting them to the poles of 
the nucleus, suggesting that the chromosomes are pulled toward the 
poles by the shortening of these fibres. As the chromosomes 
approach the pole they become less branched, less irregular in form 
and larger. On their way to the pole they seem to absorb most of 
the substance of the fibres of the chromatin spindle, drawing in the 
transverse strands of the chromatin net and taking up the substance 
of the longitudinal fibres and adding it to their mass. Thus, during 
the late anaphase, a very large proportion of all the chromatin in 
*) Pranpri (1905, 1906) believes that the equatorial portions of the spindle- 
fibres in Didinium elongate and push the chromosomes apart. 
