REPRODUCTION 433 
which is the result of the fusion of the polar nuclei, and is 
called the definitive nucleus of the embryo-sac. Each nucleus 
is surrounded by protoplasm, the egg apparatus in parti- 
cular showing three well-defined naked or primordial cells. 
The antipodal cells become clothed with cell-walls. There 
is a certain amount of protoplasm existing in the spore, 
lying around the wall and forming bridles across it, con- 
necting the peripheral substance with that in the centre in 
which the definitive nucleus is resting. 
There are no apparent archegonia: the oosphere is one 
of the three cells of the egg apparatus, the other two being 
known as the synergide. The oosphere is a product of 
the last division of the original upper nucleus, the other 
half being the polar nucleus which takes part in the fusion 
described. 
As in the Spermophytes the spore always remains enclosed 
in the ovule or sporangium, and its prothallium with the 
female organs is enclosed in it, the method of fertilisation 
of the oosphere by a free-swimming antherozoid is impractic- 
able. The problem of bringing the sexual cells together is 
met by causing the germination of the microspore to take 
place on some part of the tissue near the megaspore and by 
its prothallium taking the form of a tube which grows down 
through the tissue of the parts surrounding the megaspore to 
the megaspore itself. This tubular prothallium, known as the 
pollen tube, bears usually two male gametes, which are thus 
brought into the neighbourhood of the archegonia or the 
egg apparatus respectively. In a few species of the Gymno- 
sperms the male gametes are ciliated antherozoids, but 
usually they are two conspicuous nuclear masses asso- 
ciated with a little cytoplasm. 
In the Gymnosperms fertilisation is brought about by 
the entry of a male gamete into an archegonium. In the 
Angiosperms one of the generative nuclei fuses with the 
oosphere. In many families the other one has been seen to 
fuse with the definitive nucleus. 
After the fertilisation of the oosphere in both cases an 
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