Life-History 
25 
the layer of cells, lias to aid in expelling the anthero- 
zoids when ripe, and the cells do this by absorbing 
water rapidly, swelling, and compressing the con- 
tents of the central cell till its apex, which is not 
covered by the layer, is burst (Fig. 20), the parent 
cells are expelled, and, soon rupturing, set free the 
aiitherozoids, which have the faculty of moving 
rapidly in water or in a drop of dew or rain. 
Archegonia (Fig. 21), as previously stated, are 
the female organs, in each of which lies the 
" oosphere ; the latter, fertilised by the " anthero- 
zoa,^' becomes the oospore/^ This oospore de- 
velops into the Fern-plant bearing the well-known 
fronds, on the back of which are visible the groups 
(sori) of minute, brown spore-cases (sporangia), in 
w^hich lie numerous spores, like the one with which 
the cycle began. Thus it will be seen that the 
prothallus and the leafy Fern-plant are two gene- 
rations in the course of a single cycle. The arche- 
gonia, which are developed rather later than the 
antheridia, are situated in the middle of the lower 
surface of the prothallus, behind the notch seen in 
Fig. 17, and in the vicinity of the antheridia. Each 
of the archegonia — which are by no means so 
plentiful as the male organs, generally speaking, not 
more than two being produced on one prothallus, 
and then only one of them appearing fertile — 
originates, like the antheridia, from a cell of the 
epidermis, from which it grows out in a hemi- 
spherical form. These female organs are larger and 
more compact structures than the antheridia; their 
constitution is also totally different, for they are 
built with four tiers of cells and in a sort of columnal 
form. Their development is effected in this wise : 
A cell-wall forms, and cuts it off from the cell of the 
epidermis. It increases in size, and becomes further 
divided into three layers ; and these are again sub- 
divided by cell-walls. The result is the formation 
of a structure approximating to a flask with a long, 
narrow neck. The cavity at the base is occupied 
by a large cell, the oosphere, rich in proto- 
plasm, terminating, when mature, in a kind of style 
