SURVEY OF CELL-LIFE. 181
of the original cell is retained (independent cells), or as it is more or less completely lost (coalescing cells, and cells which undergo division).
The varieties which occur amongst the independent cells, are partly of a chemical nature, and partly have reference to a difference in the growth of the cell-membrane, by which means a change in the form of the cell may be produced.
The cell-membrane differs in respect to its chemical qualities in different kinds of cells. That of the blood-corpuscles, for instance, is dissolved by acetic acid, whilst that of the cartilage-cell is not. The chemical composition of the cell- membrane differs even in the same cell according to its age, so that a transformation of the substance of the membrane itself takes place in plants; for, according to Schleiden, the cell-membrane of the youngest cells dissolves in water, the fully-developed cells not being acted upon by that fluid. The simple cells are still more remarkable for their cell-contents. One cell forms fat, another pigment, a third etherial oil; and here also a transformation of the cell-contents takes place. A granulous precipitate is seen to form gradually in what was in the first instance a pellucid cell, and this usually takes place first around the cell-nucleus; or, vice versa, during incubation, the granulous (fatty) contents of the cells of the yelk-substance gradually undergo partial solution. According to Schleiden, this transformation of the substance of the cell-contents proceeds in accordance with a certain rule; I have not made any investigations upon the subject in animals.
We should also include under this head the formation of the secondary deposits upon the internal surface of the cell- membrane, so very frequently met with in plants. If a firm cohering substance be formed from the cell-contents, it may be deposited upon the internal surface of the cell-membrane. In plants this deposition generally takes place in layers, a stratum being formed in the first instance upon the internal surface of the cell-membrane, upon the internal surface of that one a second, and so on until at last the entire cavity may be almost filled by them. According to Valentin, these surrounding deposits always take place in spiral lines which are subject to great varieties in their arrangement, for there may be one or many of them, and they may either completely
