i68 
Davis. — Nuclear Shtdies on Pellia. 
At the time when the spirem-thread divides to form the 
chromosomes, the droplets of chromatin appear clearly in two 
rows along the side of the segments (see Figs. 26 and 27). 
The segments shorten up into rods, which clearly split longi- 
tudinally a little before their arrangement at the nuclear 
plate, and generally separate first at one end, where they are 
drawn apart, making a V-shaped figure. The dissolution of 
the nuclear membrane takes place after the spindle-fibres 
from the aster have grown down over it so as to form a sort 
of cap, as is shown in Fig. 30. When dissolved the fibres 
immediately enter the nuclear space (see Fig. 29), gathering 
the chromosomes together at the equator. There are no 
mantle-fibres, and generally very few radiations at the poles. 
The nucleolus disappears shortly before the metaphase, after 
developing a vacuolate structure and fragmenting. A new 
nucleolus is formed in the daughter-nucleus very shortly after 
mitosis. 
The thin and delicate cell-walls dividing the spore are 
plainly derived from a cell-plate of the usual structure, formed 
between the daughter-nuclei. 
There remain for our consideration the nuclear activities in 
another phase in the life-history— -the vegetative cells in the 
stalk of the sporophyte. We shall thus have examined the 
nucleus in most of the important periods of growth and 
development, excepting, however, at the time when the gametes 
are formed. The activities of the cells in the seta remind 
one of the conditions in root-tips. We find a tissue with 
very little differentiation, almost all of whose cells divide 
frequently and irregularly. 
The first indication of approaching mitosis is the elongation 
of a nucleus and an increase in the amount of chromatin, 
which takes the form of elongated bodies (see Fig. 38) that 
later unite into a much-coiled spirem. But this nucleus is 
not favourable for studies upon chromosomes, and its chief 
interest lies for us in the development of the spindle and the 
possibilities of centrosomes and asters. 
One may frequently find structures that have the appearance 
