REPRODUCTION 431 
opens a little at the apex when the archegonia are mature, 
in order that fertilisation may be possible. 
When we pass to the Phanerogams two further 
advances may be seen. The spore never escapes from the 
sporangium, and the prothallium does not emerge even in 
part from the spore, which does not open. In these plants 
the megaspore is represented by the cell known formerly 
as the embryo-sac, the sporangium being the ovwle. 
Among the Phanerogams we have two types of prothallium 
which are characteristic of the Gymnosperms and the 
5 
occ 
Fic, 177.—GERMINATION oF MEGASPORE oF Selaginella. 
arch, archegonia ; 00s, oospheres ; em', embryo. The spore has been ruptured 
and the upper portion removed. 
P 1 
Angiosperms respectively. Fig. 178 shows the structure in 
the former ; the spore or embryo-sac is filled with the pro- 
thallium, formerly called the endosperm, at the apex of 
which are several archegonia each containing a female 
gamete or oosphere. After fertilisation the resulting zygote 
gives rise to a young sporophyte or embryo, which becomes 
embedded in the endosperm. The structure thus formed, 
consisting of the sporangium or ovule, with the solitary 
spore it contains, the latter having in its interior the 
embryo surrounded by the prothallus, constitutes the 
