REPRODUCTION 439 
be illustrated by studying a series of forms. The earliest 
indication of it, which we can find, is exhibited by the 
Hydropterideew, of which Salvinia is a characteristic type. 
Salvinia is a heterosporous form, each microspore of which 
gives rise to a very rudimentary prothallium bearing only 
one antheridium with four antherozoids (fig. 175). The 
megaspore, like the microspore, is liberated from the 
sporangium, and on germination it produces a prothallium, 
part of which remains in the spore and part protrudes from 
it (fig. 176). The inclusion of the part of the gameto- 
phyte within the spore was 
probably the first step to- 
wards the formation of the 
seed. The young sporophyte 
arises upon the exposed por- 
tion of this prothallium, orig- 0 
inating as in other cases from 
the zygote produced in the 
archegonium after  fertili- 
gation. 
A more advanced stage is 
seen in Selaginella, which also 
is a member of the Pterido- 
phyta, though notafern. The 
heterospory is just as pro- 
nounced as in Salvinia. When 
the megaspore is set free from 
a 
Xe 
I> 
vy, 
ae 
Lat 
Fie. 176.—GErMination or Mraa 
the sporangium and its germi- 
nation can be observed, it is 
found that more of the game 
tophyte remains inside the 
SPORE oF Salvinia. 
pro, prothallium; a, young sporo- 
phyte. The thick wall of the spore 
has been ruptured and part of the 
prothallium is protruding, 
spore (fig. 177). The process of 
germination begins while the spore is still in the sporangium 
and by the time the spore opens the prothallium has reached 
a fair degree of development. 
A still further advance ig shown by Isoétes, in which 
the prothallium is developed inside the spore, which only 
