in Lilinm Mart agon: II. Spermatogenesis . 209 
exine, and the inmost part of it continues unthickened. Thus, 
when the pollen-grain swells up later on, there is a thin region 
in the otherwise much thickened exine, which is shaped like 
a narrow gore in a balloon (Figs. 23, 27, 31). 
The nucleus of the pollen-grain divides once before it is 
mature. It is not always easy to hit on anthers in which this 
division may be seen. Strasburger’s classical method for 
determining the stage of the pollen-mother-cell nucleus 1 is 
not applicable here, for the exine of the pollen-grain is already 
perfectly opaque, and even if the stain should penetrate, the 
nucleus would not be visible. I could not succeed in making 
the exine transparent by the use of chloral hydrate or other 
clearing agents. It is out of the question to cut sections by 
hand without previous embedding, for the pollen-grains fall 
out of the loculus at once. Thus the only plan was to embed 
anthers of different ages and cut test sections from each with 
the microtome. In this way, after many failures, I succeeded 
in finding the pollen-grain nuclei in the act of dividing. This 
occurs at a period when the anther is bright orange, but before 
it has assumed the characteristic red tint. The filament — 
hitherto so short that the anther may be called sessile — has 
just begun to grow rapidly. 
The pollen-grains in which nuclei are dividing are not 
arranged in a series within the loculus. On the contrary, one 
containing a single resting-nucleus may be found surrounded 
by others which present every stage of division. Here, as in 
the embryo-sac, there is no independent evidence by which 
to determine the seriation of stages. The nuclei are small, 
and there is some difficulty in obtaining clear differentiation 
by the ordinary staining methods. The figures which give 
detail (Figs. 25-27) are all from the same preparation, a series 
of sections 5 ju, thick cut from material fixed in alcoholic 
Hermann’s solution. In one slide of this series the staining 
has been a success, and all the details in the structure of the 
chromosomes are quite clear. The cytoplasm is very lightly 
coloured however, and for details of its structure I have 
1 Botanisches Practicum, III. Aufl. p. 609. 
