FORMATION OF CELLS. 



i7 



its centre, and the protoplasm surrounding it throws out pseudopodia which attach 

 themselves to the chlorophyll-granules in the green band and suspend the nucleus in 

 the sap-cavity.] 



Fig. 14.— Formation of the antheridium of Nitella Jlexilis (see Book II). In B the protoplasm has been 

 made to contract by reagents. 



2. The partition-^ all is formed all at once] that is, when first visible it appears as 

 a thin membrane of cellulose stretching across the whole interior of the mother-cell ; 

 the protoplasm-masses of the two daughter-cells lie in the two cavities of the mother- 



FlC. 15.— Embryos in the embryo-sac oi Allium Cepa; the cells contain very large nuclei, each with two nucleoli. In 

 / the spherical apical cell contains two nuclei (a) ; in // it has already divided [a has split up into a' and a") ; and in the 

 same manner the cell c (in /) has split up into c and c'. 



cell thus produced. This mode of cell-division is usual and perhaps even universal in 

 the formation of tissues, especially in the case of more highly organised plants. 



[The nucleus divides, and between the two new nuclei a cell-plate is formed in 

 which the cellulose wall is simultaneously produced.] (See Figs. 14 and 15.) 



\The Beha'viour of the Nucleus during Division. The general rule is that the division 

 of a cell is preceded by that of its nucleus, but to this there are certain exceptions. 

 Strasburger has found that, in the mother-cells of the spores of Anthoceros and of the 

 macrospores of Isoetes, the protoplasm divides into four parts before the nucleus shows 

 any signs of division, and that in rare instances cells of Spirogyra had divided without any 

 division of the nucleus having taken place. 



In the process of division into two the nucleus usually goes through a series of 

 changes which are designated by the term Karyokinesis : but in order that the descrip- 

 tion of the more complicated forms of karyokinesis may be intelligible, some account 



c 



