434 VEGETABLE PHYSIOLOGY 
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 cells 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 Rhodo- 
phycee and the Ascomycetes already alluded to. In both 
these groups the male gametes are the only ones that 
Y 5. kite 
Fic. 173.—DEVELOPMENT OF THE ARCHEGONIUM OF THE FERN. 
1, 2, 4, 5, 7, 8, 9, Successive stages. 3, 6 transverse sections of the 
neck region of 4 and 5. 
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 conjugation, 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 Fucacew, the members of which do not develop 
any asexual cells. While it is possible, however, for many 
