CYTOKINESIS 201 



structure which at first sight seems like a centripetally formed 

 cell-plate. The plate, however, does not extend across the centre 

 of the spindle, and is connected with the plasma-membrane at the 

 periphery. The number of fibres of the spindle now increases. 

 The furrow at its first inception is a very slender structure. During 

 quadripartition. furrows are apparently initiated within a given 

 mother-cell at one and the same time and proceed in their develop- 

 ment at approximately the same rate, so that it is the portion of 

 the central spindles between the exact central point and the exact 

 centre of the tetra-nucleate cell which is the last to be traversed 

 by the furrow. Finally, the furrows meet at the centre, dividing 

 the protoplast simultaneously into four microspores. 



Gates, working with Lathrcea, has concluded that the microspores 

 are cut off by furrows which are thickened by a special thickening 

 secreted from the protoplasm, first from the surface generally, 

 and at a later stage from the sides of the advancing furrows. At 

 the end of the process the sporocyte wall and material separating 

 the four microspores disappear and the spores are left free. 



According to Gates and Rees, in Lactuca, at the completion of 

 the homo ty pic division, constrictions appear in the cytoplasm at 

 four points placed at equal intervals on the periphery and within 

 the mother-cell wall. The interval between the cytoplasm and 

 the cell wall appears to be filled with pale-staining material. 

 These cytoplasmic constrictions now become deeper until they 

 meet in the centre and finally divide the cytoplasm into four 

 separate masses. The process of constriction may take place in 

 the absence of spindle fibres, or it may even cut across them before 

 they have disappeared, but in no case could cell-plates be observed 

 to be laid down on the spindles, the whole process taking place 

 independently of any such structure. 



Castetter has studied cytokinesis in the microsporocyte of a 

 biennial variety oiMelitotus alba. Here vacuoles play a conspicuous 

 part in the process. At the end of the second division 

 hyaline areas begin to develop between the four daughter nuclei. 

 These hyaline areas are apparently due to the movement of 

 granular material from the part toward the nuclei and is accom- 

 panied by a simultaneous extrusion of liquid into the vacuoles. 



