DIFFERENCE IN THE FORMS OF CELLS. C 



on the specific nature of the plant; — in our example it specially elongates itself 

 (Fig. 3, D and //), — whereon new changes (in this case, e.g., cell-divisions) begin. 



These examples, and many more might be added, show us that the protoplasmic 

 body forms the cell ; the cell, in the sense defined above, is evidently only a further 

 form of development of it ; the formative forces proceed from it. It has hence 

 become usual even to consider a protoplasmic body of this kind as a cell, and to 

 designate it as a naked membraneless cell or Primordial Cell ; its relationship to a 

 cell provided with membrane and cell- sap is somewhat like that of a larva to the per- 

 fect insect, which is developed from the larva into the more perfectly matured form. 



The development of a swarm-spore, Uke that of a Fucus-ovum, shows, — 

 as may also be proved in the case of every other cell, — that the substance 

 out of which the cell-wall is formed was already contained in the protoplasm in 

 some form or other which could not be recognised; and so the formation of the 

 cel]-v\^all must be regarded as a separation of matter hitherto contained in the 

 protoplasm. ' In the same manner the water of the cell-sap, although taken up 

 from without, must nevertheless pass in through the protoplasm ; and, while it 

 gathers inside as cell-sap, it takes up from it soluble substances ; and so far 

 the formation of the cell-sap also appears as a separation of matter hitherto 

 contained in the protoplasm. We shall see, further on, that the substance of the 

 nucleus also, where it is present, was originally distributed in the protoplasm, and 

 that the nucleus is formed by the collection of certain particles of protoplasm at 

 the centre of the growing cell. Thus the cell provided (by development) with 

 membrane, nucleus, and cell-sap appears as the result of a differentiation of 

 particles of matter hitherto contained in the protoplasm. The essential point is 

 this,— that this difi"erentiation always leads to the formation of concentrically dis- 

 posed layers, the outermost of which, the cell-wall, is firm and elastic, the middle 

 one, the protoplasm-sac, soft and inelastic. If the cell, as is usually the case, is at 

 first without any sap-cavity, the protoplasm is the less firm and more watery in the 

 middle, or a nucleus in this case is 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 inner cavity of the cell is always filled with actual fluid, m 

 which the nucleus often takes up a central position surrounded by protoplasm, 

 or, more usually, it approaches, together with the protoplasm, the circumference 

 of the sap-cavity, and becomes parietal. So long as that condition of cell-de- 

 velopment 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 ; it is obvious, however, that this view does 

 not apply to many true cells, e. g. to young tissue- cells (as Fig. i, A), of the true 

 nature of which we should get but a very ill-defined conception were we to regard 

 them as vesicles. The term applies still less to the structure of swarm-spores 

 and of the ova of Fuci. 



Sect. 2. Difference in the Forms of Cells.-In the conformations de- 

 scribed in the previous paragraphs, the developnjent of the cells seldom remains 

 stationary. Still further changes of form usually take place in the separate parts 

 of the cell. The collective volume of the whole cell generally increases for a 



