FORMATION AND GKOWTII OF CELLS. 



29 



10 



15 



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-groM^n. It is effected by the formation of a 



partition across the cavity of the cell, dividing it into 



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



rise to a row of connected cells, when the division 



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



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



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- 

 portant part. The nucleus, when pres- 

 ent, as it commonly is, first divides 

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

 brane, or primordial utricle, is gradu- 

 ally constricted or infolded at the line 

 of division, which, soon meeting in the 

 centre, separates the whole contents 

 into two parts by a deUcate partition ; 

 upon this a layer of cellulose is de- 

 posited as a permanent wall, which 

 Sil completes the transformation of one 

 cell into two (Fig. 21, 22). 



35. Cells multiplying in tliis way, and remaining 

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

 cording 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, weU 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, 



FIG. 10 A young cell, — the first cell of an embryo, — -with its nucleus in the centre. 

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

 12. The partition completed, so converting 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. 



FIG. 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* 



