106 THE CELL 



by a nuclear membrane, and two daughter nuclei are 

 thus constituted (Fig. 8, h). 



At the same time (in the tissue cells of plants), 

 thickenings of the achromatic spindle fibres appear in 

 the equatorial plane (Fig. 8, g), i.e. midway between 

 the two daughter nuclei, and grow till they fuse laterally, 

 forming a plate or membrane across the cell (Fig. 8, h), 

 which becomes joined to the lateral cell walls. This 

 membrane then becomes converted into cell wall sub- 

 stance, forming a new cell wall, which divides the original 

 cell into two. The achromatic spindle then disappears, 

 all except the equatorial portion on which the thicken- 

 ings which initiated the new cell wall were formed. 

 This portion of the threads, penetrating the new cell 

 wall, remain, and thus maintain continuity between 

 the protoplasm of the two sister cells. This continuity 

 can be demonstrated as existing between the protoplasm 

 of adult living cells. 



It will be seen that the process of karyokinesis has 

 the effect of dividing the chromatin of the nucleus 

 accurately into halves between the two daughter cells. 

 It is believed that each chromosome carries the organic 

 basis of certain characteristics of the cells of the par- 

 ticular species of organism, and that when each is 

 divided longitudinally into halves, these characteristics 

 are equally shared by the two halves (daughter chromo- 

 somes), and thus each daughter cell comes to have the 

 same characteristics as the mother cell. 



Development of the Adult Cell from the Embryonic 

 (Meristematic) Cell (cf. Fig. 6). So long as cells remain 

 meristematic they do not cease to pass through the 

 rhythm of growth and division, but the products 

 of cell division on the side towards the body of the 

 root or shoot gradually pass out of the meristematic 



