INTRODUCTION. 3 
latter in reference to animal organization, I here introduce an 
abridgment of Schleiden’s description of it. A delineation is 
given in plate I, fig. 1, a, a, taken from the onion. This struc- 
ture—named by R. Brown, Areola or cell-nucleus, by Schleiden, 
Cytoblast—varies in its outline between oval and circular, ac- 
cording as the solid which it forms passes from the lenticular 
into the perfectly spheroidal figure. Its colour is mostly yel- 
lowish, sometimes, however, passing into an almost silvery 
white; and in consequence of its transparency, often scarcely 
distinguishable. It is coloured by iodine, according to its 
various modifications, from a pale yellow to the darkest brown. 
Its size varies considerably, according to its age, and according 
to the plants, and the different parts of a plant in which it is 
found, from 0:0001 to 0:0022 Paris inch. Its internal struc- 
ture is granular, without, however, the granules, of which it 
consists, being very clearly distinct from each other. Its 
consistence is very variable, from such a degree of softness as 
that it almost dissolves in water, to a firmness which bears 
a considerable pressure of the compressorium without altera- 
tion of form. In addition to these peculiarities of the cyto- 
blast, already made known by Brown and Meyen, Schleiden has 
discovered in its interior a small corpuscle (see plate I, fig. 1, 4,) 
which, in the fully-developed cytoblast, looks like a thick ring, 
or a thick-walled hollow globule. It appears, however, to pre- 
sent a different appearance in different cytoblasts. Sometimes 
only the external sharply-defined circle of this rmg can be dis- 
tinguished, with a dark point in the centre,—occasionally, and 
indeed most frequently, only a sharply circumscribed spot. In 
other instances this spot is very small, and sometimes cannot 
be recognized at all. As it will frequently be necessary to speak 
of this body in the following treatise, I will for brevity’s sake 
name it the “nucleolus,” (Kernkorperchen, “nucleus-corpuscle.”) 
According to Schleiden, sometimes two, more rarely three, or, 
as he has personally informed me, even four such nucleoli occur 
in the cytoblast. Their size is very various, ranging from the 
semi-diameter of the cytoblast to the most minute point. 
The following is Schleiden’s description of the origin of the 
cells from the cytoblast. So soon as the cytoblasts have attained 
their full size, a delicate transparent vesicle, the young cell, 
rises upon their surface, and is placed upon the fiat cytoblast 
