FOEMATION AND. GKOWTII OF CELLS. 29 



tion, namely, the successive division of cells into two. This takes 

 place only when they are young and active, and mostly before they 

 are full-grown. It is effected by the formation of a 

 partition across the cavity of the cell, dividing it into C ® J 



two (Fig. 10-14). In this way, a single cell gives lo 



rise to a row of connected cells, when the division 

 takes place in one direction only; or to a plane or |fe| 



solid mass of such cells, when it takes place in two ^sj/ 



or more directions, thus producing a tissue. " 



34. In this multiplication of cells by division, as in 



the original formation of a cell, the contents and the 



protoplasmic lining play the most im- 



g^ portant part. The nucleus, when pres- 



ent, as it commonly is, first divides 



§^ into two (Fig. 11) ; then the lining mem- 

 Q brane, or primordial utricle, is gradu- 

 16 17 ally constricted or infolded at the line 



G^ ^^ division, which, soon meeting in the 

 ©O centre, separates the whole contents 



18 . ,11. 



into two parts by a deucate partition; 

 %d%3 ^|q upon this a layer of cellulose is de- 

 ©Q WM posited as a permanent wall, which 

 O© ©© completes the transformation of one 



00 ©O cell into two (Fig. 21, 22). 



35. Cells multiplying in this way, and remaining 

 united build up a row or a surface of cells, or a solid tissue, ac- 

 cordin'^ to the mode of division. But in many of the simplest 

 plants, growing in water, the cells separate as they form, and be- 

 come independent. A microscopic plant very common in shallow 

 pools in early spring, forming slimy green masses, well illustrates 

 this as shown in Figures' 15-19. At each step of this multipli- 

 cation new cell-membranes are formed, and the old one, for instance, 

 the wall of Fig. 15 and the common envelope of the two in Fig. 17, 



J'lQ. 10. Ayoung cell, — the first cell of an embryo, — -with its nucleus in the centre. 



1 1 The same, with its nucleus divided' into two, and a cross-partition beginning to form. 

 12, The partition completed, so conyerting the first cell into two. 13 The lower one again 

 divided into two, making three cells in a row. 14. The fourth cell converted into four by a 

 division in two directions, forming seven cells in all. 



FI(3. 15. A single cell, or plant of a kind of Palmella, magnified. 16. The same dividing, 

 and, 17, completely separated into two. 18. Each of these dividing in the opposite direc- 

 tion four cells are produced. 19. Each, of these again dividing into four, they produce a 

 cluster of sixteen cells. 



3* 



