ABE THE DESMIDS AND DIATOMS SIMPLE CELLS? 
131 
that the “ primordial simplicity,” which has been so constantly 
referred to as constituting their invariable characteristic, is 
altogether imaginary. Primordial simplicity may be an essential 
element in the doctrine of evolution ; but such a necessity 
does not warrant its assumption, as if it were an already 
demonstrated fact. We know that complex vital processes are 
carried on even in the lowest types of being. But because we 
neither know nor are able to conceive hoiv they are carried on, 
we are not justified in taking for granted that what appears to 
us, even with our most refined appliances, to consist of a mere 
particle of structureless jelly, must necessarily be as primordially 
simple as it appears. 
I propose on the present occasion to show that the Des- 
midiaceEe and Diatomacese — well known as these two beautiful 
groups of organisms are wherever the microscope is employed — - 
are not such simple structures as they are represented to be ; and 
that in this respect they furnish a conspicuous example of the 
danger of attaching more weight to preconceived theory than to 
actual structure as it presents itself when seen under sufficiently 
favourable conditions. But before entering on the inquiry it is 
indispensable that we should start with a clear idea of the mean- 
ing which usually attaches to the term 66 vegetable cell ” when 
applied to those lowest forms of plant life in which every indi- 
vidual cell, although constituting an integral part either of a 
symmetrically grouped series, or of a mere colony, is able to 
maintain a perfectly independent existence. 
According to the commonly accepted definition, as given in 
a recent standard work of reference, such a cell is 66 a closed sac 
or vesicle, composed of an originally imperforate membrane 
formed of the substance called cellulose, this membrane enclosing 
fluid contents so long as the cell retains its vitality ” ; the word 
“ originally” being obviously inserted in the definition with a 
view to meet the cases, constantly occurring amongst the higher 
orders of plants, in which the cell wall is more or less perforate. 
We are further told that this closed sac or vesicle is of sufficient 
strength to protect its fluid or semifluid contents ; and that it 
is formed of two quite distinct layers, the innermost of which 
(the “ Primordial Utricle ” of Mohl) is identical in composition 
with the general protoplasmic substance of the organism ; 
whereas the outer layer differs from that substance not only 
in containing no nitrogen and being closely allied in chemical 
composition to starch,* but also in not taking part in any of 
the vital processes which are carried on within the structure. 
* As a matter of fact the difference here spoken of is not so great as 
is represented, inasmuch as starch, in the shape of dextrine , is found in the 
protoplasmic contents. 
