SURVEY OF CELL-LIFE. 18] 
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 dif- 
ference 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 quali- 
ties 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 versd, durimg 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 pro- 
ceeds 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 
