INTRODUCTION 7 
yellow corpuscles found in many flowers and fruits also 
belong in this category. 
Associated with the nucleus in most animal cells, 
and sometimes found in those of plants, are certain 
small bodies, the centrospheres, each enclosing a minute 
corpuscle, the centrosome. These are usually consid- 
ered to be of great importance, especially in the process 
of nuclear division, but their absence from many plant 
cells would indicate that their importance has been over- 
rated. 
In the young plant cell the cytoplasm nearly or quite 
fills the cell, but as the latter enlarges there is an ac- 
cumulation of fluid in the cell, and this occupies the 
greater part of its bulk. This watery cell-sap is con- 
tained in cavities (vacuoles) which it has been claimed are 
integral parts of the cell, and multiply by division; but 
this view is by no means universally admitted. 
New cells may arise in several ways. The com- 
monest is by fission, or division into two parts, usually 
equal. This division is preceded by division of the 
nucleus, after which a cell-wall is formed dividing the 
cavity of the cell. (Pig. 38, D.) Less commonly there 
is a repeated division of the nucleus, followed by a 
simultaneous division of the protoplasm into as many 
parts as there are nuclei. This “internal division” is 
most common in the formation of spores and other 
reproductive cells. 
The study of the cell, and especially the changes in 
the dividing nucleus, have been the subjects of some of 
the most important researches of recent years, and have 
developed a distinct department of biology, cytology, 
with results of far-reaching importance. Nevertheless, 
