CELL FORMATION BY DIVISION. 3? 



then a partition is formed subdividing the sac ; the halves 

 either separate at once and each rounds itself off and becomes 

 an independent cell, or one or both halves again divide in a 

 similar way before they separate, and so three or four new- 

 cells are produced." 



47. In many of the filamentous Thallophytes a similar fis- 

 sion takes place, but in these the cells do not immediately sepa- 

 rate from one another after their formation. Thus, in Nostoc 

 and Oscillatoria (Fig. 26) the cells do not differ in any essen- 

 tial way as to their formation from those which constitute 

 Protococcus. In Nostoc after fission the cells round them- 

 selves up and retain but a slight and easily separable connec- 

 tion with one another ; in 



Oscillatoria, on the con- 

 trary, the cells remain cy- 

 lindrical and are less read- 

 ily separable. 



48. 111 SpirOUljra (Fig. Fig. M.-A, filament of Nottoe; B, filament 



36, p. 45) new cells form < - x aoo.-After Prami. 



by the partition of old ones. The protoplasmic sac infolds all 

 around the middle of the old cell Avhich is cylindrical in 

 shape ; into the circular channel thus formed the cell-wall 

 extends, appearing at first as a narrow projection from the 

 original wall, but becoming broader and broader, until it 

 forms a complete partition. When the new cells have 

 elongated by intercalary growth the process of fission may be 

 repeated, and so on.* 



49. The cells which make up the greater part of the 

 tissues of the higher plants are formed by fission. In the 

 apical cells of Equisetum we find a curious regularity in the 



* The student is referred to Sachs' " Text-Book," pp. 17-18, for a further 

 description of this process in Spirogyra ; and to Von Mohl's " Anatomy 

 and Physiology of the Vegetable Cell," pp. 50-51, for a description of the 

 similar fission of Cladophora glomerata (Conferva glomerata, Linn.). Von 

 Mohl's description, which was the result of the first accurate investiga- 

 tion of cell- formation, is erroneous in this that he supposes that during 

 the process, to quote his words, " a cellulose membrane is deposited all 

 over the outside of the primordial utricle" of the whole cell, and thnt 

 it is a portion of this new membrane which forms the partition. 



