CELL-DIVISION 25 



The further modification of the cell-v>all will be dealt with in 

 the next chapter. 



From the foregoing it is evident that, when cell-division takes 

 place, the nuclei of the daughter-cells are derived from that 

 of the parent-cell. This fact holds generally throughout the 

 Vegetable and Animal Kingdoms, the nucleus of any given cell 

 always being the product of a pre-existing one. The origin of 

 a new nucleus from the cytoplasm is unknown. In like manner 

 it is probable that all chloroplasts (and other kinds of plastids) 

 are derived by division from pre-existing specialised cytoplasmic 



FiG; 12. Mitosis (after Fraser and Snell). A, chromosomes have sepa- 

 rated and reached the poles. B, establishment of daughter-nuclei and 

 development of cell-plate. 



bodies. During the enlargement usually following upon cell- 

 division considerable increase of the cytoplasm may take place ; 

 but it should be emphasised that new protoplasm is only formed 

 in connection with a pre-existing protoplast, the independent 

 origin of living matter being unknown. In other words, both in 

 the Animal and Vegetable Kingdoms, one generation is merely 

 a continuation of the previous one, the reproductive cells of 

 any one dividing and enlarging to form the body of the next. 

 We can now consider how the cells produced at the growing 

 point gradually pass over into the mature condition. Just below 

 the apex they have dense cytoplasmic contents, are more or less 



