CELL-NUCLEUS—FERTILISATION 2387 
encased by the sporangial wall. The central 
cells increase, and whenever their nuclei 
divide, the full, unreduced number of chromo- 
somes can be seen, just as it may be ob- 
served in any other dividing nuclei of the fern 
plant. 
But a time arrives when the central cells 
within the sporangium become free from 
each other. Each one proceeds to grow, and 
it finally divides twice, to give rise to four 
spores. It is during these two divisions that the 
reduction in the number of chromosomes takes 
lace in the manner already described, and 
Lilies the nucleus of each spore only contains 
half the number of these nuclear structures. 
When the spores are ripe the sporangium 
bursts and the spores are scattered. If they 
happen to alight on a suitable spot, they 
germinate, but they do not bring forth a plant 
like a fern (Fig. 27). A filamentous body is 
formed which gradually develops into a heart- 
shaped green expansion known as a prothallus. 
It is very delicate, and is easily dried up, and 
consequently is only suited to live where 
conditions of moisture prevail. 
The prothallus sometimes multiplies vegeta- 
tively, by the dying off of part of the plant, 
while the living fragments grow into new 
prothalli. Sexual organs, called antheridia 
and archegonia, are developed on its under 
side. In the former a number of sperms are 
produced, while each archegonium, when 
mature, contains a single egg. We need not 
