CYTOKINESIS AND THE CELL WALL 



173 



The spindle of cytokinesis with its cell-plate commonly extends 

 laterally as a phragmoplast until it reaches the walls of the cell. In wide 

 cells with large vacuoles this may involve the formation of cytoplasmic 

 bridges (p. 151). As multinuclear endosperm becomes cellular, the 

 recently formed pairs of nuclei are connected by spindles on the flanks of 

 which additional spindles arise and so connect all the nuclei. This 

 involves a lateral extension of the spindle substance and apparently 

 also the use of any such substance remaining from the preceding mitosis. 

 Cell-plates appear in all the spindles and so subdivide the multinucleate 

 mass into uninucleate cells (Fig. 100). The development of the cellular 



-.-:.'^' 



v.. 







7 i N0.-;' 



U /; y 





r# 





if/ 



^' 



,..-^" 



\ 



V r .'4' 



Fig. 100. — Formation of cell partitions in endosperm of Iris. (After Jungers, 1931.) 



stage in cycad embryos occurs in a similar manner (Chamberlain, 1910, 

 1916). 



When one of the two telophase nuclei lies near the side of a large cell, 

 as in the angiosperm embryo sac or microspore, the phragmoplast may 

 curve sharply so that the cell-plate and subsequently formed membrane 

 cut off a small cell against the side of the large one (Fig. 125). In extreme 

 cases it may form a complete sphere about one nucleus and so cut out a 

 small cell which lies free in the cytoplasm of the original cell (Fig. 102). 

 Other modes of "free cell formation" are seen in ascospore development 

 (Fig. 88), in the delimitation of the egg in certain phycomycetes and of 

 embryo cells in certain gymnosperms (Fig. 103), and in certain animal 

 tissues (Rohde, 1923). In some cases, notably certain algse,^^ partition 

 membranes are reported to form between sister nuclei after the complete 

 disappearance of the achromatic figure. 



11 Strasburger (1892), W. T. Swingle (1897), Mottier (1900). 



