REPRODUCTION 425, 
The gametangia of the plants above the Thallophytes 
are known as antheridia and archegonia respectively. An 
archegonium is a more complex structure than an oogonium, 
being composed of many cells and showing differentiation 
into a venter and a neck (fig. 178). It contains only a 
single oosphere. 
The sexual cell differ from the great majority of 
asexual ones in never possessing cell-walls. The only 
cases in which they are clothed with them are those of the 
Rhodophycee and the Ascomycetes already alluded to. In 
both these groups the male gametes are the only ones that 
Fic. 178.—DEVELOPMENT OF THE ARCHEGONIUM OF THE FERN, 
have them; the females, as we have seen, not being 
differentiated. 
The fusion of the gametes is known as conjugation 
when they are alike, and as fertilisation when they are 
distinctly male and female. The resulting body is termed 
a zygote; it is a zygospore when it is produced by conjuga- 
tion, and an oospore when it is the result of fertilisation. 
In the more lowly organised forms it generally happens 
that both sexual and asexual reproductive cells may be 
produced upon the same individual. An exception is 
found in the Fucacea, the members of which do not 
develop any asexual cells. While it is possible, however, 
for many plants to produce both gonidia and gametes, it is 
