CELL-FORMATION. 123 



in the Cryptogamia from the Floridece upward, and in the 

 Phanerogamia) . The cells which have originated by free 

 cell-formation contain, as a rule, a lateral nucleus, rarely a 

 central nucleus (the germ-cells of certain Algse). 



V. ON CELL-EORMATION IN GENERAL. 



Now that I have discussed, in the third section of this 

 essay, parietal, and, in the fourth, free cell-formation, I 

 will compare these two processes, and thence deduce a 

 general law. 



In parietal cell-formation the contents of the parent-cell 

 divide into two or more portions. Around each of these 

 portions of cell-contents a perfect membrane is produced, 

 which, at the moment of its origin, is in contact partly with 

 the wall of the parent-cell, partly with the corresponding 

 wall of its fellow secondary cell or cells. 



In free cell-formation a greater or smaller portion of the 

 contents becomes isolated, or even the whole contents of the 

 cell. On the surface of this is formed a complete mem- 

 brane, altogether free at its outer surface (in contact neither 

 with the wall of the parent-cell nor those of its fellow 

 secondary cells. 



Cell-formation includes two stages ; the first is the 

 isolation or individualization of a portion of the contents of 

 the parent-cell ; the second consists in the origin of a 

 membrane around individualized portions of the contents. 

 Cell-formation commences with the first, and is completed 

 in the second stage. The complete individualization only 

 occurs for the express purpose of cell-formation. The 

 formation of membrane, on the contrary, is a universal 

 phenomenon, proper to the cell generally (not merely at 

 the moment of its origin). I will first speak of this latter. 



In the first half of this memoir, speaking of the cells of 

 the Algae, I have noticed the mucilaginous layer which 

 lies on the outermost boundary of the contents, and there- 

 fore close upon the surface of the cell-wall. According 



