DIV. I 



MORPHOLOGY 



25 



substance, and thus SIMULTANEOUSLY divides the mother cell into 

 two daughter cells (Fig. 13, 12 m). 



If, however, the mother cell has a large sap cavity, the connecting utricle 

 cannot at once become so extended, and the partition wall is then formed 

 SUCCESSIVELY (Fig. 16). In that case, the 

 partition wall first commences to form at the 

 point where the utricle is in contact with 

 the side walls of the mother cell (Fig. 16 A}. 

 The protoplasm then detaches itself from 

 the part of the new wall in contact with 

 the wall of the mother cell, and moves 

 gradually across until the septum is com- 

 pleted (Fig. 16 B an<2 O) ; the new wall 

 is thus built up by successive additions 

 from the protoplasm. 



In 



FIG. 16. Three stages in the division of a 

 living cell of Epipactis palustris. (x 365. 

 After TREUB.) 



the Thallophytes, even in 

 the case of uninucleate cells, the 



partition wall is not formed within connecting fibres, but arises 

 either simultaneously from a previously formed cytoplasmic plate, or 

 successively, by gradual projection inwards from the wall of the 

 mother cell. In this form of cell division the new wall commences 

 as a ring-like projection from the inside of the wall of the mother 

 cell, and gradually pushing farther into the cell finally extends com- 

 pletely across it (Figs. 17, 18). In a division of this sort, in 



f ch ck 



FIG. 17. Cell of Spirogyra in division, n, 

 One of the daughter nuclei ; w, develop- 

 ing partition wall : eft, chloroplast 

 pushed inward by the newly forming 

 wall, (x 230. After STRASBURGER.) 



FIG. 18. Portion of a dividing cell of Clado- 

 phora fracta. w, Newly forming partition 

 wall ; eft, dividing chromatophore ; k, 

 nuclei. ( x 600. After STRASBURGER.) 



uninucleate cells, nuclear division precedes cell division, and the new 

 wall is formed midway between the daughter nuclei (Fig. 17). 



In multinucleate cells a cell division does not follow on each 

 nuclear division. Among Algae and Fungi there are large and 

 externally segmented forms which consist internally of a single 



