Opalina. IST 
posterior tips of the bodies (Fig. 159, Pl. XXIII). The daughter 
cells, by attempting to swim in opposite directions, draw the poste- 
rior ends of their bodies out into slender points before they finally 
separate (cf. O. dimidiata, Figs. 307—309, Pl. XXVIII). The very 
pointed animals which thus arise (Figs. 154, 155, 160, Pl. XXIIT) can 
by this feature be distinguished from the macrogametes and the cells 
from which the latter arise. This division required in some ob- 
served instances from two to four hours, reckoning from the first 
appearance of bifurcation of the anterior end of the body until the 
complete separation. 
A second division of the same type follows. It may begin in 
one of the daughter cells before the last division is complete (cf. 
O. dimidiata, Figs. 308, Pl. XXVIII). I have never seen both 
daughter cells so dividing again before their separation. One cannot 
say how many divisions of this sort occur before the definitive mi- 
crogametes result. In the final division which forms the microgametes 
a very long and very slender thread is drawn out between the two 
daughter cells. It seems as if the animals become more sticky at 
their posterior ends and so have more difficulty in separating. One 
can watch the elongation of this thread until it becomes finer than 
one of the cilia of the body. It may reach a length more than 
twice as great as that of the body proper of one of the animals. 
Generally, on this thread, the point of original contact of the two 
bodies is indicated by a few granules resembling excretory granules 
with sometimes a little debris. One can thus determine that the 
elongation of the two bodies is frequently unequal, even as much as 
two-thirds of the thread coming from one daughter-cell. It seems 
probable that the tail of the microgamete is derived from this thread. 
In several cases in which I saw one daughter-cell dividing before 
it had completely separated from its fellow, the dividing cell had 
by far the longer tail. In each instance the undivided cell was 
somewhat pointed posteriorly, but had practically no tail (cf. O. 
dimidiata, Fig. 308, Pl. XXVIII). It is possible that the gametes 
are formed by differential division, the two sorts diverging at least 
one generation before the formation of the definitive gametes. This 
is, however, merely a suggestion with very insufficient evidence in 
its favor. . 
There are two sorts of tailed forms 1) larger ones with short 
straight tails (Figs. 154, 155, Pl. XXIII, cf. O. dimidiata, Pl. XXVIII, 
Figs. 307, 308) and 2) smaller forms with much longer tails usually 
bent at a right angle (Fig. 163, Pl. XXII). Only the latter have 
