REPRODUCTION 441 
detached from the parent sporophyte and disseminated in 
various ways. 
In the Angiosperms the formation of the seed is in the 
main similar to the process described, but it has certain 
peculiar features. The embryo-sac or megaspore has the 
same structure ag in the Gymnosperms and remains en- 
closed in the sporangium or ovule. The development of 
the prothallium is different. The megaspore has a single 
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A? 
4 
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oy 
Fre. 178.—OvuLe or Pinus, Fie. 179.—Ovurr oF AN ANQIo- 
SHOWING THE PROTHALLIUM; SPERM, SHOWING THE MEGASPORE; 
end, IN THE MEGASPORE mac; mac, WITH Its PROTHALLIUM; 
arch, archegonia, 008, OOSPHERE. 
nucleus as in other cases. When germination begins this 
divides into two, one of which travels to each end of the 
ovoid spore. Each of these gives rise by two successive 
divisions to a group of four nuclei, and a single nucleus from 
each group returns to the centre of the cell, where the two 
fuse together. These are often termed the polar nuclei. 
At this stage the prothallium ceases to undergo any change 
(fig. 179) ; it consists of a group of three nuclei at the apex, 
known as the egg apparatus; another group at the base, 
