194 A TEXT-BOOK OF BOTANY 
under surface of the prothallium in the region of the central 
axis. When the prothallia are very young, the antheridia 
begin to appear; and if the prothallia are poorly nourished 
and stunted only antheridia appear. In mature, well- 
nourished prothallia, however, archegonia also appear. In 
consequence of their late appearance, the group of archego- 
nla is near the notch, that ix, near the growing point, while 
the group of antheridia is farther back, on the older part of 
the prothallium (Fig. 186, A). 
The antheridia and the archegonia are not free and pro- 
jecting organs, as among the Bryophytes, but thev are 
more or less sunken in the tissue of the prothallium and 
open on its surface. In the case of the archegonium only 
Fre. 187.—Archegonium of a fern containing an egg (¢), the neck being eurved back- 
ward toward the antheridia. 
the neck projects, and this is usually bent backward to- 
ward the antheridia (fig. 187). The egg resembles those 
of all other archegonium-hearing plants; but the sperms 
are very different from those of Bryophytes, having large 
