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 

 //), — 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-tell, like that of the oosphere of Fucus, shows, — 

 iS does also the case of every other cell, — that the substance of the cell- wall was 

 il ready 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 diflfaren- 

 tiated porfion 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 lime, 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 



