132 Mot tier. — Mitosis in the Pollen Mothev-cells of 
Fig. 19. Multipolar spindle. The bivalents have condensed into thick lumps. 
Fig. 20, a , b. Showing manner of fragmentation of nucleolus just previous to, and during forma- 
tion of, the spindle. At b the nucleolus shows a small protuberance which is frequently seen at 
earlier stages. Near b lies a bivalent chromosome, which might easily have been interpreted as a 
mass budding off from the nucleolus. One or more bivalents are frequently found touching the 
nucleolus, a phenomenon of frequent occurrence in plants. 
Fig. 21. Outline of a cell at the stage of the multipolar spindle : the chromosomes and nucleolus 
only were drawn. This is interpreted as containing twelve bivalents and the nucleolus. At a and b 
the halves of the bivalents are not closely applied. If, however, the two members of a and b are in 
themselves bivalents, there are fourteen bivalents. The former is held to be the more probable. 
Fig. 22. Stage of the mature spindle, which often has broad poles. The chromosomes are 
unusually elongated in this cell. The usual form is shown by the chromosome at the middle. The 
two members of the chromosome at the right are probably at the point of separating, or meta- 
kinesis. An indication of the longitudinal fission of each daughter segment is visible. 
Fig. 23. Anaphase ; the daughter segments are now seen to be split lengthwise. 
Fig. 24. Cell showing the two mature daughter nuclei connected by the connecting fibres. 
Two extra-nuclear nucleoli are present in the cytoplasm. 
Fig. 25. The division of the daughter nuclei. The chromosomes have not been brought into 
the equatorial plate. When the spindle is mature, the chromosomes are arranged close together in 
a disc as in the first mitosis. Two nucleoli are present, one being derived from each daughter 
nucleus. 
Fig. 26. Telophase of second mitosis. Extra -nuclear nucleoli are present in the cytoplasm. 
Fig. 27. The four granddaughter nuclei have been formed, and all are connected by systems of 
connexion fibres. In respect to chromatin and nucleoli they all appear alike. 
Staphylea trifolia. 
Fig. 28. Pollen mother-cell showing typical nuclear reticulum with its chromatin granules and 
the large nucleolus. 
Fig. 29. Nucleus going into synapsis. The nucleolus is partly surrounded by the contracted 
reticulum. 
Fig. 30. Synapsis completed. The nucleolus is almost free from the mass. The part of the 
reticulum extending over the nucleolus indicates the manner of spireme formation from the network 
or reticulum. 
Fig. 31. The synaptic mass has loosened up considerably and the spireme has been formed. 
In the straighter portions the longitudinal fission is seen ; the halves, which are twisted about each 
other, have separated somewhat in the long stretch crossing the nucleus. At this stage the cell is 
frequently rounded off at the corners. 
Fig. 32. The spireme spreading out in the nuclear cavity. The longitudinal split, if present, is 
not well marked. This is the usual appearance of the spireme at this and the next stage, Fig. 33. 
The nucleolus lay in a neighbouring section. 
Fig. 33. The complete hollow spireme with large nucleolus. Very small darkly stained bodies 
are usually present in the cytoplasm. The cytoplasmic reticulum immediately aiound the nucleus 
is finer meshed than out near the cell-wall. 
Fig. 34. Nucleus showing second contraction. This represents a thick section of the nucleus. 
The looping is more regular and pronounced than is usually the case. 
Fig. 35. All the bivalents have been formed and a large nucleolus is still present. The weft 
of kinoplasmic fibres running parallel with the contour of the nucleus is present midway between 
the latter and the cell-wall. 
Fig- 36. Tangential section of nucleus showing bivalents. The long piece of the spireme may 
represent more than one bivalent. 
Fig- 37- The same stage in radial section. The form of the bivalents is typical. 
Fig- 38, a, b, c. Three sections including an entire nucleus in the same stage as the preceding 
figure. In a two bivalents have been pushed out of the nucleus in the preparation of the section. 
These three figures show the variation in the size of the bivalents and the different disposition of the 
two members of each other that may exist in the same nucleus. 
