SELF-DIVISION OF CELLS. 189 
The single-celled organisms, that is, those which during 
life retain the form of a single cell, for example the Ameebe, 
as a rule propagate themselves in the simplest way by self- 
division. This process differs from the previously described 
self-division of the Moneron only in the fact that at the 
commencement the firmer cell-kernel (nucleus) falls into two 
halves, by a pinching in at its middle, The two young ker- 
nels separate from each other and act now as two distinct 
centres of attraction upon the surrgunding softer albu- 
minous matter, that is, the cell-substance (protoplasma). By 
this process finally the latter also divides into two halves, and 
there now exist two new cells, which are like the mother cell. 
If the cell was surrounded by a membrane, this either does 
not divide at all, as in the case of egg-cleavage (Fig. 3, 4), or it 
passively follows the active pinching in of the protoplasm ; 
or, lastly, every new cell exudes a new membrane for itself. 
The non-independent cells which remain united in commu- 
nities or states, and thus constitute the body of higher or- 
ganisms, are propagated in the same manner as are inde- 
pendent single-celled organisms, for example, Amceba (Fig. 2). 
Just as in that case, the cell with which most animals 
and plants commence their individual existence, namely, the 
egg, multiplies itself by simple division. When an ani- 
mal, for instance a mammal (Fig. 3, 4), develops out of an 
Fic. 3.—Egg of a mammal (a simple cell). 
a. The small kernel speck or nucleolus (the so- 
called germ-spot of the egg). b. Kernel or 
nucleus (the so-called germ-bladder of the egg). 
c. Cell-substance or protoplasm (the so-called 
yolk of the ege). d. Cell-capsule or membrane 
(membrane of the yolk) of the egg; called in 
mammals, on account of its transparency, Mem- 
brana pellucida. 
