4 INTRODUCTION. 
like a watch-glass upon a watch. It is at this time so delicate 
that it dissolves in distilled water in a few minutes. It gradu- 
ally expands, becomes more consistent, and at length so large, 
that the cytoblast appears only as a small body inclosed in one 
of the side walls. The portion of the cell-wall which covers the 
cytoblast on the inner side, is, however, extremely delicate and 
gelatinous, and only in rare instances to be observed; it soon 
undergoes absorption together with the cytoblast, which like- 
wise becomes absorbed in the fully-developed cell. The cyto- 
blasts are formed free within a cell, in a mass of mucus-granules, 
and the young cells lie also free in the parent cell, and assume, 
as they become flattened against each other, the polyhedral 
form. Subsequently the parent cell becomes absorbed. (See a 
delineation of young cells within parent cells, plate I, fig. 2, 
6, 6,6.) It cannot at present be stated with certainty that the 
formation of new cells always takes place from a cystoblast, and 
always within the existing cells, for the Cryptogamia have not 
as yet been examined in this respect, nor has Schleiden yet ex- 
pressed his views in reference to the Cambium. Moreover, 
according to Mirbel, a formation of new cells on the outside of 
the previous ones takes place in the intercellular canals and on 
the surface of the plant in the Phanerogamia. (See Mirbel on 
“ Marchantia,” in Annales du Musée, 1, 55; and the counter- 
observations of Schleiden, Miiller’s Archiv, 1838, p. 161.) A 
mode of formation of new cells, different from the above de- 
seribed, is exhibited in the multiplication of cells by division of 
the existing ones ; in this case partition-walls grow across the 
old cell, if, as Schleiden supposes, this be not an illusion, inas- 
much as the young cells might escape observation in conse- 
quence of their transparency, and at a later stage, their line 
of contact would be regarded as the partition wall of the parent 
cell. 
The expansion of the cell when formed, is, either regular on 
all sides, in which case it remains globular, or it becomes poly- 
hedral from flattening against the neighbouring cells, or it is irre- 
gular from the cell growing more vigorously in one or in several 
directions. What was formerly called the fibrous tissue, which 
contains remarkably elongated cells, is formed in this manner. 
These fibres also become branched, when different points of 
the cell-wall expand in different directions. This expansion of 
