DIFFERENCE IN THE FORM OF CELLS. 
5 
rest, it also changes its form, and increases in volume, cell-sap collecting in the 
interior. The cell formed in this way now grows in a manner dependent on the 
specific nature of the plant ; — in our example it elongates itself (Fig. 3, D and 
H), — and new changes (in this case cell-divisions) begin. 
These examples — and many more might be added — show us that the proto- 
plasm constitutes the cell ; the cell, in the sense defined above, is evidently only a 
further development of it ; the formative forces proceed from it. It has hence 
become usual to consider a protoplasm-mass of this kind as a cell, and to designate 
it as a naked membraneless or Primordial Cell. 
The development of a swarm-cell, like that of the oosphere of Fticus, shows, — 
as does also the case of every other cell, — that the substance of the cell-wall was 
already contained in the protoplasm in some form or other which could not be 
recognised; the formation of the cell-wall must be regarded as a separation of 
matter already existing in the protoplasm. In the same manner the water of the 
cell-sap, although taken up from without, must nevertheless pass through the proto- 
plasm; and, while it collects inside it as cell-sap, it takes up from it soluble sub- 
stances ; so far the formation of the cell-sap is also a separation of matter hitherto 
contained in the protoplasm. The nucleus is probably to be regarded as a differen- 
tiated portion of the protoplasm. Thus the mature cell, provided with cell-wall, 
nucleus, and cell-sap, is the result of a differentiation of matter already contained in 
the protoplasm. The essential point is this, — that this differentiation always leads 
to the formation of concentrically disposed layers, the outer of which, the cell-wall, 
is firm and elastic, the inner, the protoplasm-sac, soft and inelastic. If the cell, as 
is usually the case, is at first without any sap-cavity, the protoplasm is less firm and 
more watery in the centre, or a nucleus is in this case formed, which, at least in 
young cells, is always more watery than the surrounding protoplasm. When at last 
the cell-sap makes its appearance, the cavity of the cell is always filled with actual 
fluid, in which the nucleus often takes up a central position surrounded by proto- 
plasm, or, more usually, approaches the circumference of the sap-cavity, and becomes 
parietal. So long as the phase of cell-development in which the cell appears as 
a sap-cavity bounded by a membrane— certainly the one most commonly seen — had 
alone been observed, it was correct enough to define the cell as a vesicle ; but it is 
obvious that this view does not apply to many true cells, e.g. to young cells 
which form component parts of a tissue (as Fig. i. A), of the true nature of 
which we should get but an ill-defined conception were we to regard them as 
vesicles. The term applies still less to swarm-cells and to the oospheres of 
Fucus. 
Sect. 2. Difference in the Form of Cells. — The development of the indi- 
vidual cell by no means always results in the forms just described ; further changes 
of form usually take place in the separate parts of the cell. The volume of the 
entire cell generally increases for a considerable time, with corresponding increase 
of the cell-sap ; not unfrequently it amounts to a hundred or even a thousandfold 
the original volume. During this increase, the form commonly changes; if it was 
at first roundish or polyhedral, it may become filiform, prismatic, or tabular, or 
branch into a number of arms. The cell-wall may increase very considerably in 
