668 Graham . — Nuclear Division of Preissia commit tat a. 
chromosomes and partly in the formation of the central spindle fibres. It 
is generally accepted that in many Thallophytes the nucleolus often con- 
tributes a portion of its substance to the formation of the chromosomes, 
and it would not be surprising if in some of the Liverworts the nucleolus 
also contained some of the chromatin. 
Indications of a cell plate appear in the earliest telophase observed 
(Fig. 23). In later telophase the cell plate, in section, appears as a line of 
granules. These increase in number until a distinct plate is formed (Figs. 24, 
25, 26). At first the cell plate lies only between the two daughter nuclei, 
finally extending its circumference to the periphery of the cell. The 
formation of the cell plate in Preissia commutata agrees with the formation 
of the cell plate in the vegetative cells of Pellia epiphylla described by 
Farmer and Reeves (’94), and in the vegetative cells of the archegoniophore 
of Marchantia polymorpha described by Van Hook (’00). 
Discussion. 
Although a number of papers on mitosis in the gametophyte of the 
Liverworts have appeared, many of them were concerned primarily with the 
centrosphere or centrosome problem. Farmer and Reeves (’94) describe 
centrospheres in the germinating spores of Pellia epiphylla ; Farmer (’95) 
also describes them in the germinating spores of Fegatella conica ( Cono - 
cephalum conicum ). Van Hook (’00) reports them associated with centro- 
somes in cells in the archegoniophore of Marchantia polymorpha. 
Chamberlain (’03) describes the presence of asters in the prophase division 
in the germinating spores of Pellia epiphylla , their disappearance at meta- 
phase, reappearance at telophase, and second disappearance with (see p. 33) 
the formation of the nuclear membrane. Usually they have nearly dis- 
appeared at the time of the third division. Davis (’01) describes prominent 
asters in the first three mitoses of the germinating spores of Pellia epiphylla , 
but they are less plainly shown in the second and third division. He does 
not find them in the stalk of the sporophyte. There is, therefore, agreement 
between these investigators as to the presence of centrospheres or asters in 
cell-division during certain stages of development of the vegetative cells of 
some Liverworts. The fact that asters are prominent in the first division 
of the germinating spore of Pellia epiphylla , that they become less prominent 
in the second and third division, finally disappearing, makes it reasonable 
to expect that they may not be formed during cell-division in the growing 
thallus of some Liverworts. The same may be said with reference to the 
presence of centrosomes. Both are surely absent from all phases of mitoses 
in the cells of the thallus of Preissia commutata , as far as my observations 
are concerned. So many cell generations lie between the mitoses in the 
germinating spore and the fully formed thallus that, if they are present in 
