CELL DIVISION 13 



(see Fig. 5). All of the spores thus produced lie unconnected 

 within the mother cell. This is known as free cell formation. 



In Spirogyra and other cases among the Thallophytes the 

 dividing cell-wall is produced by a gradual ingrowth from the 

 wall of the mother cell between the two daughter nuclei which 



A 



FlG. 5. Free cell formation of spores in the ascus of Erysiphe communis. A, ascus 

 with single nucleus; C, cytoplasm; N, nucleus; NL, nucleolus; B, successive stages in 

 nuclear division within the ascus; at X, early anaphase, nuclear membrane, NM, still 

 persisting; R, kinoplasmic radiations from the poles; at Y, telophase, new nuclear membrane 

 not yet formed; Z, a later stage where the nuclear membranes demark the daughter nuclei; 

 C, A, B and C, are successively later stages than Z in B. At A some of the kinoplasmic 

 radiations are bending downward about the nucleus TV; at B the nucleus is nearly enclosed; 

 and at C entirely enclosed by the radiations, which now form a complete membrane cutting 

 off a portion of the cytoplasm of the ascus, and thus forming a complete cell. (Arranged 

 after drawings by Harper. B and C are diagrammatic. For the sake of simplicity of 

 description, various stages of nuclear division are shown in a single ascus, although at any 

 given time only one stage would actually be present.) 



have previously been formed. This process of wall formation 

 reminds one of gradual closing in from all sides of an iris 

 diaphragm. 



In yeasts and some other fungi cell division is preceded by 

 budding. The mother cell puts forth an outgrowth like itself 

 in general form; the nucleus divides and one of the daughter 

 nuclei enters the bud; and a division wall is then formed sepa- 

 rating the bud from the parent cell (Fig. 6). 



