THE BLUE ALG 473 
which all the stages in the life-history of the individual may 
often be observed. 
We need a few technical terms to designate such parts as 
have just been described. A mass of protoplasm capable of 
more or less individual activity is called a cell.: All plants, 
and all animals as well, consist of one or more cells. Usually 
in plants the protoplasm is inclosed by a cellulose envelope 
known as the cell-wall, all within which is then distinguished 
as the cell-contents. Since a cell-wall implies at least the 
previous existence of living cell-contents, the term cell may 
be applied even to the empty chamber from which all life 
has gone. In the living protoplasm a comparatively large, 
dense kernel, more or less clearly marked off, is termed the 
nucleus,? the rest of the protoplasm being distinguished as 
the cytoplasm; * while any liquid part of the cell-contents is 
called cell-sap; and the entire protoplasmic part, a protoplast. 
The process through which one cell becomes two by enlarging 
and splitting in halves is known as fission.‘ 
Successive fissions often take place in such a way that the 
partitions are in planes at right angles to one another, with 
the result, shown in the tint-balls, that more or less cubical 
groups of cells are formed—an arrangement which sometimes 
passes into a globular or irregular one through changes in 
the direction of growth or division. If instead of forming 
partitions at various angles, the cleaving planes are always 
parallel, so that successive fissions are in the same direction, 
then we have a chain or row of cells. This is what happens 
in the colonies of alge known as “‘fallen stars’? (Nostoc, 
Fig. 306), because of the sudden appearance of their glisten- 
ing balls when swollen by rain. Here numerous blue-green 
cells, like beads on a string, are embedded in a copious mass 
of jelly secreted by the protoplasm; for instead of forming 
distinct cell-walls this mucilaginous cellulose, for the most 
part, becomes homogeneously fused. At intervals in the 
1Cell < L. cella, a small room or hut. 
2 Nu’cle-us < L.a little nut or kernel < nuz, nut. 
3 Cy’to-plasm < Gr. kytos, a hollow or cell. Originally a synonym 
of protoplasm, the word cytoplasm has now taken on the restricted 
sense above defined. 
4 Fis’sion < L. fissio, a dividing. 
