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 



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 Confervaceae and Desmidiese, 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 



