32 Prof. H. Karsten on the Formation, 
their relative position, there is an analogy with the cells of 
Cladophora; in these latter, however, the distinction is less 
marked. 
In the case of Spirogyra no transitional forms are discover- 
able betwixt the central colourless cells and the peripheral cells 
or vesicles filled with chlorophyll and aggregated into or con- 
tained in a sac. Moreover these two kinds of secretion-cells 
are met with in all the other Confervaceze and Desmidiez, and it 
is upon their disposition in the mother cells that the peculiar 
marking of these organisms, which frequently serves for charac- 
terizing the genera and species, depends. 
But further, these two varieties of secretion-cells occur not 
only in the tissue-cells of these simple plants, but also in the 
complex tissue of higher plants, where they take part in the 
assimilation of nutrient matter derived from without—the one 
variety, frequently colourless, containing hydrocarbons, the 
other, usually coloured, filled with nitrogenous compounds. 
§ IX. 
The structure and development of the nucleus (nuclear cell): its multipli- 
cation by endogenous cells.—Circulation of the cell-juices between the 
secretion-cells from the walls of the mother cell to the nucleus. 
Particular attention has always been devoted to the cell- 
nucleus in the centre of the joint-cell of Spirogyra, and in this 
case, as elsewhere, a particular function in the multiplication of 
the cell has been ascribed to it. 
The production of the cell-nucleus, which, in general, like 
that of the cell itself, is referred to the division of preexisting 
nuclei and to their new formation from the contents of the 
mother cell, and which is supposed constantly to precede the 
production of the membrane of the developed cells (whether this 
takes place by constriction of the wall of the mother cell or by 
free-cell formation in the cell-juice), is ascribed, in the case of 
Spirogyra, to the division of the preexistent nucleus of the 
mother cell. 
As regards the notion of the division of the cell-nucleus, in 
the first place, the same error prevails in this as with respect to 
cell-multiplication itself. The existing nucleus is divided neither 
by the sudden appearance of a delicate membrane stretched 
across the radius of the nucleus nor by folds growing inwards 
from its membrane, but by the production of new cells by the 
side of its endogenous cell, the nucleolus, which under these 
circumstances itself contains a nucleolar corpuscle, and thus 
becomes the nucleus of the nuclear cell. 
Soon after the first appearance of the daughter cells produced 
in the lentiform or discoid cell-nucleus, these are found at the 
