1 90 Sargant. — Formation of the Sexual Nuclei 
aqueous fixing-solutions. Details of these processes are given 
in the Appendix at the end. 
Hand-sections have proved of great value for purposes of 
comparison with thin serial sections in the study of the first 
division of the pollen-mother-cell nucleus, but they have not 
been used to illustrate the two following divisions. The 
history of the fourth nuclear division in this series — that 
which takes place in the pollen-tube — has been worked out 
almost entirely by means of hand-sections from alcohol- 
material. This is the only part of the work in which a single 
fixing method has been used to the exclusion of any other. 
Formation of the Nucleus of the Pollen-grain. 
The anther of Lilium Martagon differs from the ovule in 
possessing a definite archesporial tissue. This grows rapidly 
by repeated cell-division from the time of its first formation 
until the pollen-mother-cells are formed. Cell-division then 
ceases in the loculi for some time, during which the nuclei 
of the pollen-mother-cells pass through a period of growth 
and development which corresponds in every detail to that 
following on the differentiation of the embryo-sac nucleus. 
The nuclear divisions in the archesporial tissue are pre- 
cisely similar to the vegetative divisions described in the 
first part of this paper (I, p. 451). The chromosomes are 
somewhat crowded on a small spindle, and it is therefore 
difficult to find examples in which they can be counted with 
certainty, but there are always about twenty-four. We may 
conclude that when the pollen-mother-cell is differentiated, 
its nucleus is built up of twenty-four chromosomes. 
In an ordinary flower-head of Lilium Martagon , in which 
ten or twelve buds are still sessile and crowded together at 
the top of the flower-stalk, anthers from the youngest buds 
will usually show archesporial tissue in which there is no 
lack of nuclear divisions. I possess a series of preparations 
from ten buds of such an inflorescence. The whole flower- 
head originally consisted of twelve buds, two of which were 
