MITOSIS (KARYOKINESIS) 21 
width until it has completed the separation of the two 
protoplasmic masses, this being the commoner way in 
the lower plants, or the wall may be produced sim- 
ultaneously at all points at the plane of separation be- 
tween the two protoplasts, as is the case in most higher 
plants. In some of the lower plants the whole wall be- 
gins to constrict at the middle, giving the appearance of 
pinching the cell into two separate cells which are then 
free from one another. A peculiar type of fission is that 
termed budding, in which a small outgrowth appears at a 
point on the cell, gradually enlarging until it is as large 
as the old cell and then becoming separated from it by 
constriction of the wall at the point of emergence. This 
is especially characteristic of, but not confined to, some of 
the yeasts, 
33. Cell division is in most cases initiated by, or more 
or less immediately preceded by, the division of the 
nucleus. In coenocytes, on the contrary, this connection 
between nuclear division and that of the coenocyte seems 
to be lacking. Two types of nuclear division may be 
distinguished, direct or amitotic and indirect or mitotic. 
The latter process is generally known as mitosis or karyo- 
kinesis. The direct division is comparatively rare and 
appears to consist of a simple pinching in two of the nu- 
cleus. By far the commonest method is that of mitosis. 
This is a very complicated process and is essentially as 
follows, being subject, however, to many more or less 
pronounced variations in different plants. If a centro- 
some is present, which is apparently the case only in some 
of the lower plants, it divides into two centrosomes which 
move around outside the nucleus until they lie at oppo- 
site sides in a line at right angles to the plane of division. 
The nuclear reticulum now begins to resolve itself into a 
fine tangled thread without cross connections, the chro- 
