CYTODS AND CELLS. 13! 



individual always consists of two different parts, the outer 

 protoplasm and the inner nucleus. 



In order to be thoroughly convinced that every cell is 

 an independent organism, it is only necessary to trace the 

 active phenomena and the development of one of these tiny 

 bodies. We then see that it performs all the essential life- 

 functions which the entire organism accomplishes. Every 

 one of these little beings grows and feeds itself indepen- 

 dently. It assimilates juices from without, absorbing them 

 from the surrounding fluid; the naked cells can even 

 take up solid particles at any point of their surface, and 

 therefore eat without using any mouth or stomach. 

 (Cf. Fig. 15.) Each separate cell is also able to re- 

 produce itself and to increase (Fig. 8). This increase 

 generally takes place by simple division, the nucleus parting 

 first, by a contraction round its circumference, into two 

 parts; after which the protoplasm likewise separates into 

 two divisions. The single cell is also able to move and 



(T"() 



FIG. 8. Blood-cells, which increase by 

 division, from the embryo of a young stag. 

 Each blood-cell has originally a kernel, and is 

 globular (a). When they are about to in. 

 crease, the cell-kernel, or nucleus, first separ- 

 ates into two kernels (b, c, d). The protoplas- 

 mic body then becomes pinched in at a point 

 between the two kernels, which become more 

 widely separated from each other (e) . Finally 

 a complete separation between the two parts 

 is effected at the point where the original cell 

 was pinched in, so that there are now two 

 cells (/). (After Frey.) 



creep about, if it has room for free motion, and is not pre- 

 vented by a solid covering ; from its outer surface, it sends 



