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I 



The Nucleolus, a granular body enclosed in a 

 hyahne matrix and surrounded by a wall of nucleoplasm, is 

 comparatively large in young cells, and undergoes partial 

 or complete dissolution during early mitosis. It is r^laced 

 in later stages by new ones, which become the nucleoli of 

 the daughter nuclei, or some may be dispersed in the cyto- 

 plasm and disappear. An extreme form, probably of 

 nuclear synapsis, in pollen mother-cells and embryo-sac 

 mother-cells gives condensed nucleolus-like nuclei, similar 

 to those observed in spirogyra or spermatozoids. 



Mitosis, or indirect nuclear division, is of two 

 kinds:— Heterotype, characteristic of spore mother-cells; 

 and homotype, of all other cell nuclei. The leading 

 features are described under four stages. PROPHASE: 

 dealing with changes in the nuclear reticulum, spirem (and 

 synapsis— heterotype or reduction division), chromosome 

 formation, spindle and nuclear-plate ; Metaphase : the 

 longitudinal splitting of each chromosome in the nuclear- 

 plate ; Anaphase : separation of the daughter chromo- 

 somes, and their movements from the equatorial region of 

 the cell to its opposite poles ; TELOPHASE : massing of the 

 chromosomes at the poles, their union into a temporar>- 

 spirem (heterotype process) or into the network of a resting 

 nucleus (homotype). The heterotype division is reducing, 

 the daughter nuclei containing half the somatic number of 

 chromosomes, and characterises the change from sporo- 

 phyte to gametophyte in the alternation of generations. 

 It may be a qualitative division, the homotype is 

 equational. In Monocotyledons the pollen mother-cells are 

 separated by a wall after each mitosis— successive division : 

 in Dicotyledons the four pollen cells are formed before they 

 are parted by walls— simultaneous division. 



AmiTOSIS, direct division of the nucleus, is shown in 

 cells dividing under exceptional conditions. It occurs lu 



