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INTRODUCTION TO CYTOLOGY 



plast into two portions. In other cases a delicate membrane appears in 

 the cytoplasm and develops into a partition between the two resulting 

 cells. In the meristems of higher plants this membrane commonly forms 

 before the spindle substance has disappeared from the equatorial region, 

 so that the spindle seems to give rise to it. 



In the last mentioned case the formation of a "cell-plate" appears 

 to be a feature of the mitotic process, but it should be pointed out that 

 this is probably because cytokinesis follows mitosis so closely. All 

 degrees of correlation between these two processes are known. No 

 correlation whatever is seen in certain plasmodial masses, cleavage fur- 

 rows developing through the cytoplasm without any evident relation 

 to the nuclei. In other cases the division of the cytosome is in some way 



Fig. 58. — Diagram of a typical case of nuclear division and cell-division in animals. 



related to nuclear influence but not to the mitotic process. Thus cleav- 

 age furrows or membranes may develop in positions clearly dependent 

 upon the positions of the nuclei, the result being definitely uninucleate 

 cells rather than the irregular multinucleate blocks seen in certain cases 

 of plasmodial division. Frequently, as in the budding of yeast and the 

 division of certain sporocytes, division of the cytosome follows so closely 

 upon nuclear division that it seems in some way to depend upon it, 

 without, however, involving the mitotic mechanism itself. Finally, 

 cytokinesis may be so intimately connected with mitosis that the two 

 are like a single process, the presence of the spindle between the nuclei 

 causing a special series of changes in the plane of cytokinesis. In the 

 interest of accuracy, however, the term "mitosis" should be applied only 

 to nuclear division; it is not synonymous with "cell-division." 



Cell-division in Animals. — Cell-division in animals is fundamentally 

 similar to that in plants. Typically it differs, however, in the presence 

 of centrosomes and in the mode of cytokinesis (Figs. 58, 59). As mitosis 



