96 C. CLIFFORD DOBELL. 
and 19, Pl. 4. They are roughly spherical, ovoid, or some- 
times of the form shown in fig. 19, with a notch at one end. 
This notch marks the former position of the cytostome. In 
fully-formed cysts the nucleus can be clearly seen asa re- 
fringent sphere, lying in the middle of the protoplasm. 
The second case (2), in which the zygote does not encyst, 
is of considerable interest. After swimming about for a time, 
the monad—which, although really a zygote, is indistinguish- 
able from the asexually-reproducing animals—comes to rest 
and divides in the normal way. It can be seen to contain 
but one nucleus (which stained preparations show to be de- 
rived from the fusion of the two gamete nuclei), although the 
manner in which this single nucleus arose I have not been 
able to observe satisfactorily in the living animal. On one 
occasion when I carefully watched the conjugation of a pair 
of monads and their subsequent development I found that 
three hours and ten minutes elapsed from the time when com- 
plete fusion of the gametes had taken place until the zygote 
began to divide longitudinally. 
The zygotes and their descendants appear to be able to 
continue dividing for a considerable period, though I cannot 
say definitely how long. In cultures in which most of the 
monads appeared to have conjugated—though one cannot be 
certain that all have done so—I have often found a few divid- 
ing individuals a week or ten days afterwards. In the end, 
however, the majority of these appear to encyst, the flagellum 
being drawn in and the cvst wall formed as in the case of 
the monads which encyst immediately after ‘conjugation. 
(See Pl. 4, figs. 31-33). 
As far as I am aware there is but one flagellate monad which 
has had ascribed to it the ability of continuing to lead an 
active life after conjugation. I refer to Dunaliella, one of 
the Phytomonadina, whose life-history has been investigated 
by Teodoresco (50). He describes the fusion of isogametes. 
One loses its flagella and is absorbed. Under favourable 
conditions encystment does not follow fusion, but the monads 
remain active. The nuclear phenomena have not been 
