Cytology of Polyphagus Euglenae . 189 
into two groups. The structure of the nuclei at the fusion stage has not 
been clearly ascertained, but there are indications that previous to fusion 
the chromatin mass breaks up into granules, probably chromosomes 
(Figs. 8o, 81). The subsequent stages of nuclear division in the sexual 
sporangium seem to follow the same order as in the asexual sporangium, 
and similar changes can be observed in the final segregation of the zoospores, 
but the details have not been followed out in stained specimens. Fig. 83 
shows a nucleus in what appears to be an early division stage ; Fig. 84 
shows the division of the primary nucleus of the sporangium into two, and 
Fig. 85 a late stage in the division of two nuclei. The number of chromo- 
somes in each daughter nucleus could not be counted, but they appeared to 
be not greater than ten. 
Nuclear Division. 
Nuclear division takes place only in the sporangia, never in the vegeta- 
tive cells, gametes, or zygotes. The single large nucleus (Figs. 26-2 8) 
which enters the asexual sporangium from the vegetative cell at once 
divides into two, then four, eight, sixteen, thirty-two, sixty-four, &c., until 
a very large number are formed, sometimes several hundred. The number 
produced varies within very wide limits from two or four in very small 
sporangia to several hundred in the larger ones. 
I have not been able to observe the process of division of the primary 
nucleus of the sporangium, but in all the later stages it is mitotic, and 
is probably the same, therefore, in the primary nucleus. The spindle 
appears inside the nucleus before the wall is broken down (Figs. 30, 33, 
38-40), and is apparently formed out of the lightly stained chromatin mass 
which is found in the middle of the nucleus in contact with the chromatin 
cap. The chromosomes are about ten or twelve in number and very minute 
(Figs. 30, 33, 38). Only a very small portion of the large and dense 
chromatin mass is used up in their formation ; the rest forms a thick 
peripheral layer on the wall of the nucleus (Figs. 30-39), which is visible 
during all the stages in the prophases of division, and is finally set free in 
the cytoplasm to be used up in the further growth of the sporangium. 
Soon after the appearance of the spindle the nuclear wall at the poles 
of the nucleus disappears (Figs. 33, 40), and the rest of the nuclear wall con- 
tracts and becomes flattened (Fig. 39). The two poles of the spindle 
protrude slightly through the polar openings into the cytoplasm. At this 
stage centrosome-like structures become visible at the two poles (Fig. 39). 
The daughter groups of chromosomes then separate to the opposite poles 
of the spindle (Figs. 41, 42), the nuclear wall disappears entirely, and the 
peripheral mass of chromatin which was in contact with it contracts into 
a more or less globular mass, which is left lying in the cytoplasm (Figs. 31, 
34 , 35 > 4 2 )- The chromosomes become aggregated at the periphery of the 
p 
