222 Sargant . — Formation of the Sexual Nuclei 
plate x. 
Figs. 1-15. First karyokinesis in pollen-mother -cell. 
Fig. 1. Pollen-mother-cell soon after its formation, with resting-nucleus. X585. 
Fig. 1 a. Resting-nucleus from older mother-cell : n, nucleolus. At x the 
thread shows dots. x 1000. 
Fig. 2. Pollen-mother-cell with nucleus in which first signs of approaching 
synapsis are seen. The chromatic network begins to contract away from the 
nuclear wall to the right of the figure. Besides the vacuolated nucleolus, there is 
a shapeless mass of nucleolar matter to the left, which is green in the thicker part 
about x , but washy red at the edges. X585. 
Fig. 2 a. Bit of thread from nucleus of same age as that shown in Fig. 2, more 
highly magnified. The thread is irregularly thickened, and shows two rows of 
dots. At n, n , drops of nucleolar matter are attached to it. 
Fig. 3. Pollen-mother-cell with nucleus in synapsis. A general view of the 
whole cell from a hand-section. The nucleolus is hidden by the close coils of the 
contracted chromatic thread, x 585. 
Fig. 3 a. Section from nucleus rather older than that drawn in Fig. 3 : n, the 
nucleolus, now recovering its shape. There is still a good deal of amorphous 
nucleolar matter among the coils of the contracted chromatic thread, x 585. 
Fig. 3 b. Bit of thread (from place marked x in Fig. 3 a), more highly magni- 
fied. It is now of uniform breadth and the double row of dots is clear. 
Fig. 4. Pollen-mother-cell with spirem-nucleus. From a hand-section. X585. 
Fig. 4 a. Tangential section from nucleus rather older than that of Fig. 4. 
The ribbon begins to be articulated, x 1000. 
Fig. 5. Pollen-mother-cell with nucleus in which the spirem-thread has begun 
to split longitudinally (x x') : n, nucleolus, x 585. 
Fig. 5 a. Tangential section from nucleus in which the splitting of the ribbon 
is almost complete, x 1000. 
Fig. 6. Pollen-mother-cell in which the spirem-ribbon of the nucleus has begun 
to fall into lengths, as at x . Longitudinal fission is already complete : n , 
nucleolus. From the same hand-section as Fig. 5. X585. 
Fig. 7. Pollen-mother-cell with nucleus, in which the twelve double filaments 
formed from the spirem-ribbon have begun to cluster round the half-dissolved 
nucleolus. The nuclear membrane is indistinct, and much of the sap is coloured by 
the nucleolus. X585. 
Fig. 7 a. Section of nucleus nearly the same age as that shown in Fig. 7. 
From a series of thick sections (15/4) from alcohol-material. All the phenomena 
of synapsis have reappeared. x 1000. 
Fig. 8. Pollen-mother-cell from same section as Fig. 7. The optical section of 
the nucleus probably passes through the smallest diameter of the nucleus. The 
filaments are shorter and thicker; probably each consists of a double row of dots, 
but the section is too thick to make this certain. The nucleolus assumes its 
definite outline and the nuclear membrane reappears, x 585. 
Fig. 8 a. Part of a single pair of filaments from a nucleus rather older than that 
shown in Fig. 8. Each filament is irregularly thickened and bears a double row 
of dots. Magnified about 1000 times. 
Fig. 9. Pollen-mother-cell from a thin serial section. The immature chromo- 
somes consist of two twisted lengths of linin-ribbon, each bearing two rows of 
