Lecture XX VI II. 253 



halves part asunder and moving along their associated fibres migrate 

 to the opposite poles. Round the poles the half chromosomes con- 

 gregate and draw together to form a dense mass of chromatin and 

 linin. 



While the halves of the bisected chromosomes are moving apart, 

 at the middle of each fibre of the achromatic spindle, a bright 

 refringent speck appears. It is composed of a cellulose-like sub- 

 stance called pectose. The specks of pectose together form the cell- 

 plate. They gradually grow in size and finally coalesce to form 

 a disc, still apparently perforated by fine filaments of protoplasm 

 the persisting remains of the achromatic fibres. Subsequently 

 layers of cellulose are deposited on each side of the cell-plate 

 and are extended out from it to unite with the lateral walls of the 

 cell, thus dividing it into two. While this partition-wall is being 

 constructed the masses of chromosomes at the poles become more 

 regular by the drawing in of those limbs of the chromosomes 

 which were still projecting into the cytoplasm, or were delayed 

 in their motion along the fibres. As soon as, or even before, 

 the surface of the mass is smoothened by this means, vacuoles 

 or cavities appear in its substance. Vacuolation continues, the 

 partitions dividing the vacuoles become reduced to filaments and 

 only an extremely delicate pellicle remains to separate the coalescing 

 vacuoles from the surrounding cytoplasm. Nucleoli reappear 

 and the two nuclei resulting from the division of the original 

 one are completely reconstituted. By this time the new cell- 

 wall is complete and the new cells, each with its own nucleus, have 

 been produced from the original single cell. Before the next 

 division of the new cells their nuclei will divide again in the 

 manner just described. 



The process of nuclear division which I have just outlined 

 is called mitosis or karyokinesis. In it the following stages are 

 for convenience of description usually recognised : (i) The spireme- 

 stage, when the nuclear thread is freed of its anastomoses and 

 forms a continuous skein in the nuclear cavity ; (2) the equatorial 

 plate, nuclear plate or monaster stage, when the chromosomes lie 

 in the equatorial plane of the nucleus and appear as a stellate mass 

 when viewed from the poles ; (3) the diaster stage, when the longi- 

 tudinal halves of the original chromosomes, having parted asunder, 

 come to form two stellate masses round the poles ; (4) the 

 dispireme stage, when the nuclear net-work is reformed in the 

 two resulting nuclei. 



Evidently by the process of mitosis the ever-growing material of 

 the nucleus of* the oosperm is allotted equally to all the cells of the 



