in Lilium M art agon : II. Spermatogenesis. 191 
accidentally destroyed during the process of preparation. 
The lowest buds in the head were shortly stalked, and stood 
somewhat apart from the others. The outer perianth-leaves 
of each bud were broken off, and the rest of it was cut into 
a series of transverse sections in which all the parts are 
shown in their proper position. Archesporial nuclear divisions 
are frequent in the anthers of the two youngest buds. In the 
third the pollen- mother-cells are differentiated, and the first 
indication of an embryo-sac can be seen in the median ovules 
(I, p. 449). Sections from the lower buds show every stage 
in the history of the pollen-mother-cell nucleus, from its 
first formation to the differentiation of the spirem-ribbon 
within it. 
Such a series of preparations serves two purposes. It 
may be considered as a sort of index to the nuclear stages 
included within it, giving direct evidence as to their seriation ; 
and if we have any means of determining the “difference 
of age between buds at the beginning and those at the end of 
the series, we know at once about how long the nuclei of any 
particular tissue in those buds have taken to pass from one 
stage to another. I should suppose that such a bud as the 
third in our series would take a week or ten days to attain 
the size of the lowest bud, and this must also be the time 
taken by the pollen-mother-cell nuclei to develop from the 
resting to the spirem-stage. 
The whole interval between the complete differentiation 
of the pollen-mother-cell and the formation of the young 
pollen-grains within it may conveniently be divided into 
four periods. 
1. The nucleus of the pollen-mother-cell grows larger and 
alters in structure, finally assuming the well-known spirem- 
condition (Figs. 1 -3#). 
2. Twelve chromosomes are formed from the spirem-ribbon, 
and lie loose in the nuclear cavity (Figs. 4-10). 
3. The first karyokinesis of the pollen-mother-cell nucleus 
separates the two halves of each chromosome. Cell-division 
follows this nuclear division (Figs. 10 a-\ 5). 
