VEGETABLE CYTOLOGY 73 



iinin threads of the nuclear reticulum shorten, drawing the chro- 

 matin granules together into a thickened twisted chromatic thread. 

 This thread splits transversely and thus becomes divided into a 

 number of rods termed chromosomes. Each of these then splits into 

 two longitudinal halves that may be termed the daughter -chromo- 

 somes. They lie within the nuclear cavity united by delicate threads. 

 There now begins a phenomenon concerned with the cytoplasm 

 which is primarily a process of spindle formation. The granular 

 cytoplasm accumulates at the poles of the elongated nucleus forming 

 the cytoplasmic caps. Presently it begins to show a fibrillar struc- 

 ture, the threads extending outward around the periphery of the 

 nucleus forming an umbrella-like arrangement of fibers from both 

 cytoplasmic caps. With the formation of fibers comes a breaking 

 down of the nuclear membrane and in consequence the fibers enter 

 the nuclear cavity and organize the spindle. Some of the fibers 

 become attached to the split chromosomes and push, draw or pull 

 them to the equatorial plate, halfway between the poles. Mean- 

 while the nucleolus disappears. As the chromosomes line up at the 

 equatorial plate their daughter halves are drawn apart in V-shaped 

 fashion. The split extends and eventually one daughter-chromo- 

 some is drawn to one pole and the remaining half to the other. At 

 the respective poles the daughter chromosomes form a dense com- 

 pact knot. A cell membrane, composed of material contributed 

 largely through the shrinking of the spindle fibers, is now formed 

 through the middle of the spindle. This soon splits to form a thin 

 vacuole lying between the two membranes (the plasma membranes) . 

 Presently there appears within the vacuole and between the mem- 

 branes a carbohydrate substance. On either side of this deposit the 

 plasma membranes form a cellulose membrane. The flattened 

 vacuole extends toward the periphery and ultimately a complete 

 cell wall is formed. 



The dense compact knots of chromosomes at the poles of the 

 spindle, that are to form the daughter-nuclei, now begin to expand 

 and clear mesh-like spaces to appear between the expanding threads. 

 As this process advances the chromosome substance becomes dis- 

 tributed throughout the nuclear cavity. It is soon possible to dis- 

 tinguish the chromatin from the Iinin. Eventually an irregular 



