THE ACHROMATIC FIGURE, CYTOKINESIS, AND CELL WALL 187 



whole equatorial plane at the same time, and not as a progressive in- 

 growth from the periphery. 



In the fungi Harper and others showed that the two daughter cells are 

 separated by the development of a cleavage furrow in which the new wall 

 is laid down. In large multinucleate masses that become broken up into 

 spores this progressive cleavage is a very complicated process. The 

 manner in which the furrows develop is shown in the studies of Timberlake 

 (1902) on Hydrodictyon, D. B. Swingle (1903) and Moreau (1913) on 

 Rhizopus and Phy corny ces, Davis (1903) on Saprolegnia, Rytz (1907) 

 on Synchytrium, and Harper (1899, 1900, 1914) on Synchytrium, Pilobolus, 

 Sporodinia, Fuligo, and Didymium. In Rhizopus (Fig. 63, A) the 

 cleavage furrows begin to form both at the peripheral membrane of the 

 sporangium and at the columella and work gradually into the multi- 

 nucleate protoplasm, eventually cutting out multinucleate blocks which 

 become the spores. In Phycomyces (Fig. 63, B) small vacuoles appear in 

 the midst of the multinucleate protoplasm, enlarge and become stellate, 

 and cut out spore masses with from 1 to 12 nuclei 

 each. In the myxomycete, Fuligo, the cleavage 

 is from the surface inward, and the multinucleate 

 blocks are subdivided by further furrowing into 

 uninucleate spores. In Didymium the spores are 

 delimited in a similar way by furrows which 

 begin to form along the young capillitium fila- 

 ments in the interior of the multinucleate mass 

 as well as at its periphery. 



Microsporocytes. In the microsporocytes of 

 the higher plants it has been shown with great 

 clearness by Farr (1916, 1918) that the quadri- 

 partition to form spore tetrads of the tetrahedral 

 type is brought about by furrowing, previous ac- 

 counts having generally stated that the walls 

 are formed by the cell plate method. Farr finds 

 that after the four microspore nuclei are formed 

 they all become connected by a series of six 

 spindles, or sets of connecting fibers. The two 

 spindles of the second maturation mitosis may 

 persist, four new ones being added, or the two 

 may disappear, six new ones being developed. 

 Although some sporadic thickenings may ap- 

 pear on these fibers they have nothing to do with the formation of the 

 separating walls, there being no centrifugally growing cell plates such as 

 are seen in cells dividing by the cell plate method. Constriction fur- 

 rows appear at the periphery of the cell (Fig. 64) and grow inward until 

 they meet at the center, dividing the protoplast simultaneously into four 



FIG. 64. Cytokinesis by 

 furrowing in the micro- 

 sporocyte of Nicotiana. 

 X 1400. (After Farr, 1916.) 



