530 
Thomas.—Double Fertilization in a 
get so big that only a few slender strands of protoplasm 
bridge the central cavity. 
The polar nuclei have generally completely fused before 
the entrance of the pollen-tube, but there is evidently a 
considerable amount of latitude in point of time, for I have 
found polar fusion in embryo-sacs of very different ages. 
The greater number of cases have been seen in sacs of the 
same age as that drawn in Fig. I, or slightly older. Here 
the ovule is evidently quite young, for the integuments have 
not yet closed over the nucellus, while within the embryo-sac 
all the nuclei, even those of the antipodals, are in good 
condition, and all possess, as a rule, one central nucleolus. 
The antipodals are large pear-shaped cells and, at this period, 
very often contain two nuclei apiece produced by karyo- 
kinesis, but later, when the centre of the sac is occupied by 
one large definitive nucleus, the nuclei of each antipodal cell 
have run together to form a large irregular mass, containing 
many nucleoli. Frequently, however, the ovule is much 
older than this at the time of polar fusion, and in one instance 
the pollen-tube was seen in the micropyle at that period. 
This did not mean the early arrival of the pollen-tube, for 
the contents of the embryo-sac showed signs of age. Its 
cavity was immensely larger than that of the sac from which 
Fig. i was drawn, and its oosphere had increased in size 
while the synergids were somewhat degenerated; the anti¬ 
podals also were bigger, and their centre occupied by a large 
deeply-stained nuclear mass. The coalescence of the polar 
nuclei is evidently taking place very late, for endosperm and 
embryos show most commonly in the other embryo-sacs 
of this ovary. 
When the generative nuclei are extruded from the pollen- 
tube 1 they are very minute and stain heavily. Very often 
they are merely oblong, or lens-shaped, but sometimes they 
1 The swollen end of the pollen-tube is provided with a sharp point, evidently 
for boring through the nucellar tissue. I have seen the pollen-tube turn at right 
angles when passing through the nucellar tissue so that it entered the embryo-sac 
laterally. 
