CYTOKINESIS AND THE CELL WALL 171 



Alterations in surface tension and viscosity, together with proto- 

 plasmic streaming, are obviously important factors in cytokinesis of 

 certain types, but comparatively little is known about the initial causes 

 of these phenomena. That they are in some way associated with changes 

 in the permeability of protoplasmic membranes seems clear from the 

 work of Spek and others on animal eggs and of Stalfelt (1921) on plant 

 cells. Little more than a promising beginning has been made on the 

 problem of the mechanism of cytokinesis. 



Closely allied to the process of furrowing is the type of cytokinesis 

 seen in certain tissues which are more or less plasmodial in their early 

 stages of development. In some forms of cartilage and epithelium, for 

 example, vacuole-like masses of material appear in the cytoplasm and 

 gradually subdivide the Plasmodium into cells which, however, remain 

 connected as a syncytium (Fig. 10). In certain cases such masses are 

 metaplasmic in nature (Rohde, see p. 45). 



Cjrtokinesis by Cell-plates. — In the bryophytes and vascular plants 

 cytokinesis in somatic cells commonly begins with the formation of a 

 cell-plate through the equatorial plane of the spindle substance lying 

 between the young sister nuclei. The same process occurs in some sporo- 

 cytes also. The precise manner in which the cell-plate is formed and 

 becomes involved in the developing cell wall has long been a subject of 

 dispute. According to many of the earlier accounts,* the cell-plate is 

 formed by the fusion of swellings on the middle portions of persisting 

 spindle fibers, the plate then splitting to form the plasma membranes of 

 the two daughter protoplasts, the primary wall layer, or "middle lamella," 

 then being secreted between them. 



In a series of more recent papers^ it is emphasized that after adequate 

 fixation (Bouin, Benda) the cell-plate first appears as a continuous film 

 in the midst of the oriented "spindle of cytokinesis" and not as a series 

 of swellings or granules. This film proceeds to thicken, often unevenly 

 at first. The "splitting of the plate" so often observed is held to be due 

 to improper fixation, for it is said to appear only after the use of certain 

 fluids and not in unfixed cells (Robyns, 1929). Belaf (19296), in his 

 ingenious experimental studies on living cells of young Tradescantia 

 leaves, finds that the young cell-plate is a fluid layer offering less resistance 

 to shrinking agents than the spindle, so that the halves of the cell can 

 be made to round up and separate along this plane before a primary cell 

 membrane is present. Hence he concludes that the cell-plate is really a 

 transverse cleft in the spindle, the membrane substance being deposited 



^ E.g., Strasburger (1898), Timberlake (1900), and C. E. Allen (1901). 



9 Devise (1922) on the sporocytes of Larix; Robyns (1924, 1926, 1929) on the root 

 cells of Hyacinthus, Viola, and other plants; Martens (1927c, 19296) on stigma cells 

 of Arrhenatherum and stamen hairs of Tradescantia; Jungers (1931) on the endosperm 

 of Iris. 



