SELF-DIVISION OF CELLS. 1 89 



The single-celled organisms, that is, those which during 

 life retain the form of a single cell, for example the Amoebse, 

 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 surrounding 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, Amoeba (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 



Fig. 3. — Egg of a mammal (a simple cell). 

 a. The small kernel speck or nucleolus (the so- 

 called germ-spot of the egg), h. Kernel or 

 nucleus (the so-called germ -bladder of the egg). 

 c. Cell-substance or protoplasm (the so-called 

 yolk of the egg), d. Cell-capsule or membrane 

 (membrane of the yolk) of the egg ; called in 

 mammals, on account of its transparency, Mem- 

 brana pellucida. 



