CHAP, iv.] from 1838 to 1851. 333
outer face. There are two processes in the formation of a cell ; the first is the isolation or individualising of a part of the contents of the mother-cell, the second the formation of a membrane round the individualised portion.' He then proceeds to show that the cell-wall is formed by the separation of non- nitrogenous molecules from the nitrogenous mucilage (proto- plasm). These sentences contain all that is general and essential in vegetative cell-formation. Further on he notices the peculiarities in the various processes in cell-formation ; he says that the individualising of the cell-contents assumes four forms ; first, single small portions of the contents separate themselves inside the rest, as occurs in the formation of free germ-cells in Algae, Fungi, and Lichens, and of endosperm- cells in Phanerogams ; secondly, the whole contents of one cell, or of two by conjugation of associated cells, collect into a free spherical or ellipsoidal mass, as in the formation of germ- cells in the Conjugatae ; thirdly, the whole contents of a cell separate into two or more portions, which is now called cell- division ; from this N�geli distinguishes as his fourth form, the process known as abscision (Abschniirung), which occurs in the formation of germ-cells in many Algae and Fungi.
Schleiden had declared it to be a general law in plants, that cells are only formed inside mother-cells. Meyen however, Endlicher, and Unger, had recently assumed the formation of new cells between the older ones ; N�geli maintained that all normal cell-formation, vegetative and reproductive, takes place only within mother-cells.
In opposition to the long-cherished notion that there must be one general and fundamental form of cell, N�geli pointed to the fact that cells have very different forms at the moment of their production. Those which arise by free cell-formation are, he says, at first always spherical or ellip- soidal ; those produced by cell-division have a shape neces- sarily conditioned by the form of the mother-cell and the
manner of division. He showed further that changes in the