816 QUARTERLY CHRONICLE OF MICROSCOPICAL SCIENCE. 
some species a diccious condition exists, which reaches back 
to the formation of the zoospores. Some of these, instead of 
producing the normal plant, produce a form of a very modified 
character (‘dwarf-males’), which attach themselves to the 
filaments containing oospheres, and merely serves the purpose 
of developing antherozoids (fig. 11). The oospore produces 
a succession of asexual generations. The zoospores germi- 
nate and form a filament which eventually becomes septate. 
Sometimes, however, while still in the unicellular condition, 
it discharges its contents as a zoospore by a terminal opening 
in a manner which suggests a comparison with Vaucheria. 
That, however, is more probably related to Saprolegniee 
than a reduced form of Gidogonium. 
Alternation of generations.—The comparison of the different 
groups of Thallophytes requires that the relationship of the 
sexual and asexual generations of an organism should be 
taken into account. In Nostoc the asexual generations 
succeed one another uniformly; in Sperogyra there is equal 
uniformity in the succesion of generations which are sexual. 
But generally the two kinds are intermixed in varying pro- 
portions. In the simplest alternation a sexual and an 
asexual generation follow each other with perfect regularity. 
In others the sexual generations are intercalated at more or 
less remote intervals in a series the other terms of which are 
asexual. Thisis what happens in Gidogonium. It may easily 
happen that if the sexual generation makes its appearance at 
very distant periods it may for a long time remain unobserved. 
This may be the reason that it has not yet been detected in 
any of the Siphophycee, except Vaucheria. 
An important feature in the life-history of any organism 
consists in the similarity or dissimilarity of the two kinds of 
generations which arise from sexual and asexual reproduc- 
tion. In Vaucheria, for example, there is no difference 
between the character of the sexual and asexual plants. 
But such a case as a moss is a conspicuous example of the 
loss of all vegetative characters by the asexual generation 
which is reduced to a mere mass of spores. These, collec- 
tively, are due to a sexual process of which each, therefore, 
is only a partial product. ‘The sporocarp of mosses represents 
a type of alternation of generations where an asexual one is 
so reduced as to have lost all individualization, and has become 
merely a means of extending as widely as possible the effects 
of the sexual act in the other generation. Good examples of 
this arrangement are afforded by Cystopus, where the oospore 
developes from its contents a number of zoospores ; Spheroplea 
