224 A TEXTBOOK OF GENERAL BOTANY 
The nucleus (Fig. 229, 4) is usually rounded and is surrounded 
by a membrane known as the nuclear membrane. Within this 
there is a clear substance, the nuclear sap or karyolymph, which 
is not readily stained. Embedded in the karyolymph is a net- 
work, or reticulum. This is usually said to be composed of linin 
on which another substance, chromatin, is distributed. Chromatin 
is very easily stained, so that it is conspicuous in stained prepa- 
rations. The nucleus also usually contains one or more rounded 
bodies, the nucleoli. The function of the nucleoli is somewhat 
obscure, and it is probable that they have various functions in 
different cells. In general, however, the nucleolus would seem 
to be a mass of accumulated material which is usually, though 
not always, utilized in the metabolic processes of the nucleus. 
Mitosis. The division of the cell is initiated by the division 
of the nucleus. In the ordinary method of division a nucleus 
passes through many stages, and the whole complicated process 
is known as mitosis. Its chief function seems to be to divide 
all parts of the chromatin equally between the two daughter 
nuclei. Mitosis may be considered as consisting of the following 
stages: prophase, metaphase, anaphase, and telophase. 
Prophase. The initiation of the division of the nucleus is 
marked by the resolving of the reticulum into a number of 
more or less distinct units, the chromosomes (Fig. 229, B, C, D, £). 
These chromosomes are frequently arranged end to end in a 
more or less continuous thread (syireme), which later segments 
transversely into separate chromosomes. Sometimes, however, 
the chromosomes are independent from their first formation. 
The material of the chromosomes condenses and becomes more 
or less evenly distributed along the length of the chromosomes. 
This results in giving them a dense appearance. The chromo- 
somes may be variously shaped, but they are perhaps most 
frequently somewhat long and slender. 
While the chromosomes are being formed, fibrils make their 
appearance in the protoplasm surrounding the nucleus. These 
fibrils assume such a position that they radiate from two points, 
called poles, on opposite sides of the nucleus. The nuclear 
