ANATOMY OF THE CELL. Zl 
among lower plants it has.only been traced with certainty in | 
the older cells of the Characeae. The process consists in the 
elongation of the nucleus, the central portion becoming small 
or thread-like, and the two halves stretching apart until the 
thread breaks and the two parts both assume the regular form 
of the nucleus. Such a process has been observed in the cells 
of Tradescantia and in the bast cells of Urtica urens. 
4. Indirect Nuclear Division, or Karyokinesis. 
The different phases through which the nucleus passes in 
this method of division has been studied in various plants. As 
some stages appear much more clearly in certain plants than in 
others, the descriptions of the process of karyokinesis in the 
‘plant cell are generally taken from several sources and com- 
bined in order to illustrate the entire process. For the first 
steps, the nuclear division in the embryo-sack of /ritillaria 
impertalis furnishes a good illustration. Pollen mother-cells of 
different plants show better the formation of the cellulose wall. 
This is usually considered the final step in the process of 
karyokinesis, but it is not true that indirect nuclear division is 
always accompanied by cell division. 
Nuclei, when about to divide, are large, well-defined bodies 
possessing either a single nucleolus or several, and containing 
also a substance distinguished from the remaining contents by 
its form, which is either that of small distinct granules scattered 
in the cytoplasm, or that of a combination of these granules in 
such a way as to form a network of fine threads. This is 
the part of the nucleus known as chromatin, or that which takes 
the dark color when staining fluids are used. The first appear- 
ance of division in Fritillaria is the formation of this substance 
into a number of short thick threads, called nuclear segments. 
In other cases, these segments are simply irregular masses of 
less definite shape. (See Fig. 15, 2.) Shortly after this for- 
