farr: cytokinesis of poixen-mother-cells 289 



the length of one edge of the tetrahedron, as is shown on the upper 

 side of figure 6; or they may be more abrupt, as indicated on 

 the lower side of the same drawing. In no case was the inner edge 

 of the furrow found to be sharp. In cells in which the protoplast 

 has collapsed and shrunken into a smaller ball, the wall retains 

 the form which it had at the time that the collapse occurred 

 (fig. 7). In such cases the appearance is as of ridges on the inner 

 surface of the mother-cell-wall. The presence of these ridges in 

 the fresh living material and their persistence, after plasmolysis 

 has drawn the turgid protoplasm away, demonstrates that they are 

 in no sense artifacts, or related to plasmolysis in any way, but 

 that they actually occur in the division of the cell. 



The furrowing of the plasma membrane and the simultaneous 

 invagination of the mother-wall take place more rapidly at certain 

 places than at others. It is evident that if there are four nuclei 

 arranged tetrahedrally and equidistant from each other within a 

 sphere, there will be four points upon the surface of that sphere 

 which are equidistant from each of three of the nuclei. These 

 points are the centers of the faces of the tetrahedron above de- 

 scribed. By the transformation of the sphere into the tetrahedron 

 the plasma membrane has been brought closer to the nuclear 

 membrane. At these four points above mentioned on the plasma 

 membrane the constriction of the mother-cell continues to 

 proceed more rapidly than elsewhere. Thus there are formed 

 four projections of the inner surface of the cell-wall which are 

 equidistant from each other, one being in the center of each 

 face of the tetrahedron, and which are connected with each other 

 by a ridge of less magnitude. These projections, or invaginations, 

 continue to elongate toward the center of the 'cell, and conse- 

 quently in the direction of the fourth nucleus, keeping at all 

 times equidistant from the three nuclei. As a result the four 

 projections meet in the center of the tetranucleate cell, and fusion 

 of their tips occurs (fig. 8). Thus there are organized four 

 protoplasmic masses each with a single nucleus and connected 

 with each of the other four by an isthmus of cytoplasm, at first 

 quite broad. It is evident that there would be six such isthmuses, 

 not more than three of which would show in any one optical 

 section. 



Apparently each isthmus constricts independently of the others. 



