174 NATURAL SCIENCE. Marcu, 
living thing presented itself as a sum of vital unities, any one of 
which manifested the properties of life. 
The living matter of the cell was held to be sui generis. Not-. 
living matter could not, by any combination we knew of, apart from: 
the genetic intervention of living things, take on its unique properties.. 
Only from the living could the living arise. This living matter was, 
and is, called protoplasm; and even now a cell can hardly be more 
clearly defined than as a small mass of protoplasm enclosing a 
peculiarly organised element known as the nucleus. 
The further study of cells, aided by the constantly increasing 
power and precision of our microscopes, revealed complexity, reticu- 
lation, striation, foliation, radiation, and other appearances, at 
present undetermined by the best observers with the finest accessible 
optical powers ; while the nucleus is seen to be highly complex, and 
subject tothe most striking internal modifications during its cyclic 
changes. 
To pursue or even indicate the history of the prolonged and un- 
tiring work upon the cell during recent years is no part of the object 
of this paper; enough to recall that the nucleus is found to be the 
initiating centre of all the great changes which the cell undergoes. 
But the study of the cell has been conducted in the main (1) upon 
cells belonging to highly complex organisms, and, therefore, as vital 
unities of that organism must partake, in all their mutations, of the 
complexities involved in the developmental organism of which they 
are units; and (2) the different processes of cellular change and 
nuclear modification have been studied, of necessity, after death, and 
under the influence of desiccation and staining. 
For years it has impressed itself upon the mind that the great 
problem of the nature and behaviour of the cell should at least be 
begun where the problems to be solved ought to present themselves. 
in their simplest condition, and that would appear to be amongst the 
unicellular organisms—the organisms whose complete life, when traced 
through all its cyclic changes, begins in a cell and ends in a cell, pre- 
senting an elementary though, in their case, a permanent condition 
in the evolutional history of more complex organic forms. 
These are easily and everywhere accessible in that remarkable 
but minute group of organisms known as Saprophytes. They succeed 
the saprophytic Bacteria in the destructive fermentation of large 
masses of dead organic tissue. There are only a few of these forms 
known, and their successive action upon the decomposing mass is 
both mechanical and physiological. 
They are strictly unicellular; the majority are nucleated cells; 
but the problem of the cell is rendered the more interesting by the 
study of them, from the fact that amongst the group are distinctly 
non-nucleated forms.* 
1 Vide the author’s researches, Month. Micro. Journ., vols. x.,xi., xii., xiii., xiv., xvi. ; 
also Proc. Roy. Soc., 1878, and Journ. Roy. Micro. Soc., ser. ii., vols. v. and vi. 
