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 Nageli 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 ; Nageli 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, Nageli 

 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 



