693 
of Fertilization in Angiosperms. 
the preliminary work of cutting ovaries of different ages, and 
looking over the series of sections to determine which contain 
embryo-sacs at the exact point of development, is in itself 
heavy. Outside the Liliaceae and their allies -we rarely find 
ovules arranged in rouleaux, and must sometimes sacrifice 
a whole ovary to secure median sections of a single ovule. 
Nor are adjacent ovules necessarily in the same phase of 
growth: fertilized and unfertilized are found in the same 
ovary, and among the fertilized some are too young and some 
too old for our purpose. In many cases it is essential to 
work with microtome sections only: in all they must be used 
to determine the minute structure with precision. But there 
are disadvantages in sections so thin as those of microtome 
series. The embryo-sac is commonly cut into many slices, 
of which each must be observed with a very high power, and 
the solid body mentally reconstructed from the succession of 
layers. The labour of thus observing a number of embryo- 
sacs, the sections of which are found in groups among long 
series of useless sections extending over many separate slides, 
and of indexing them in order to record such observations, 
can hardly be exaggerated. 
Nor is the interpretation of results without difficulty. In 
some of these species the generative nuclei do not show the 
vermiform shape which makes it easy to distinguish them 
from the other nuclei in the embryo-sac. This is the case, 
for example, in Endymion nutans (Guignard, 4). Both 
the generative nuclei in this plant are small and egg-shaped. 
In some species, too, the polar nuclei are completely fused 
into one by the time the pollen-tube enters the embryo-sac. 
Now in plants which combine these features the identification 
of the second generative nucleus must of necessity be laborious. 
In the first place, a number of preparations from an earlier 
stage of development will be required to show that :the fusion 
of the polar nuclei is commonly complete before the entrance 
of the pollen-tube. Even when this is established, a single 
preparation showing the fusion of two similar nuclei in the 
centre of an embryo-sac, which also contains a fertilized egg- 
