CELL-DIVISION 25 



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

 the next chapter. 



From the foregoing it is e\ident that, when cell-di\-ision takes 

 place, the nuclei of the daughter-ceUs 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 hke manner 

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

 are'derived bj- division from pre-existing speciahsed cytoplasmic 





B 



V 



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 usuall}' foUomng upon ceU- 

 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 li\-ing matter being unknown. In other words, both in 

 the Animal and \'egetable Kingdoms, one generation is merely 

 a continuation of the previous one, the reproductive cells of 

 any one di\'iding and enlarging to form the body of the next. 

 We can now consider how the cells produced at the gro\T,ing 

 point gradually pass over into the mature condition. Just below 

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



