186 



INTRODUCTION TO CYTOLOGY 



CYTOKINESIS 



In the foregoing pages discussion has been limited largely to karyo- 

 kinesis. In the present section attention will be directed to cytokinesis, 

 or the division of the extra-nuclear portion of the cell. 



In plants the wall separating the two daughter cells is formed by 

 two general methods: cell plate formation and furrowing. The first and 

 more common of these methods, by which a wall is formed in close 

 association with the spindle fibers at the close of mitosis, has been briefly 

 described in the foregoing section on the achromatic figure (p. 176) and 

 will be taken up in greater detail in the following section on the cell wall 

 (p. 190). At this point we shall therefore describe the second method, 

 that of furrowing, which in plants is seen most conspicuously in the 

 thallophytes and in the microsporocytes of the higher plants. The 

 review of the subject given by Farr (1916) will be followed. 



Thallophytes. In Spirogyra Strasburger (1875) showed that the 

 wall between the two daughter cells appears as a " girdle" or ring-like 

 ingrowth from the side wall of the parent cell. This wall continues to 



FIG. 62. FIG- 63. 



FIG. 62. Cytokinesis by furrowing in Closterium. Only the central part of the cell 

 is shown. X 700. (After Lutman, 1911.) 



FIG. 63. 



A, Cleavage furrows beginning to form at periphery of sporangium of Rhizopus nigricans. 

 X 1500. B, Cleavage in the sporangium of Phycomyces nitens: intersporal substance in the 

 angular furrows. X 500. (Both after D. B. Swingle, 1903.) 



grow centripetally by the addition of new material at its inner edge while 

 the protoplast develops a deep cleavage furrow, the process continuing 

 until the separating wall is completed at the center of the cell. A 

 somewhat similar process occurs in Closterium (Lutman 1911) (Fig. 62). 

 In the brown algae Sphacelaria (Strasburger 1892; W. T. Swingle 1897) 

 and Dictyota (Mottier 1900) the wall develops uniformly across the 



