2 INTRODUCTION. 
division of the nucleus. The first two of these processes will be dis- 
cussed under zuclear division, while the third will be dealt with in 
connection with cell-formation. 
NUCLEAR DIVISION. 
KARYOKINESIS IN CELLS OF THE LOWER PLANTS IN WHICH 
CENTROSOMES AND CENTROSPHERES ARE DEVELOPED. 
At present there are recognized two types of development of the 
karyokinetic spindle. In one the spindle arises through the instru- 
mentality of individualized dynamic centers or centrospheres, as in 
certain Thallophyta and Liverworts; in the other, it is developed 
wholly independently and in the absence of any such centers, as, for 
example, in the higher plants. We speak of types of spindle develop- 
ment in this connection also for the sake of convenience, since centro- 
spheres have not been found in connection with the development of 
the spindle in all Thallophytes; but the author does maintain that 
centrospheres have not been demonstrated to occur in any plant 
above the Bryophytes, and that in the Angiosperms such structures 
do not in all probability exist. 
As illustrating the development of the spindle in which centro- 
spheres are present, the tetraspore mother-cell in Dictyota dichotoma 
will be selected from the alge and the mother-cell of the ascus in 
Lrysiphe from the fungi. 
It is not considered necessary, nor conducive to any better under- 
standing of the facts presented here, to enter into any lengthy dis- 
cussion concerning the structure of the firmer framework of the 
cytoplasm. The consensus of opinion now is that the firmer substance 
of cytoplasm consists of either a reticulum of fibrillz or of an alveolar 
or foam structure (Waben of German literature) and that, in many 
cells, these two structures intergrade into one another. 
DICTYOTA. 
The cytoplasm of the tetraspore mother-cell of Dictyota dichotoma 
during the preparation for nuclear division presents two well-defined 
portions, the kinoplasm, which is always associated with the nucleus 
and plays the most important réle in the karyokinetic process, and the 
remaining alveolar portion. Numerous chloroplasts are also present. 
The first indication of mitosis is the appearance, on opposite sides 
of the nucleus, of two large sharply defined asters of kinoplasmic 
fibers radiating from a rod-shaped body, which is often slightly bent, 
lying either close to the nuclear membrane or at some little distance 
from it (Fig. 1, A). The rod-shaped body is the centrosome, which 
