350 Harper.—Sexual Reproduction in Pyronema 
Meanwhile conspicuous changes have been taking place in 
the oogonium. When mature the oogonium was filled with 
very densely staining protoplasm, and its nuclei were evenly 
distributed through its whole interior (Figs. 6, 7, and 10), but 
with the migration of the antheridial nuclei into the conjuga¬ 
tion-tube, the nuclei of the oogonium begin to migrate toward 
its centre (Fig. 14) where they become collected in the most 
typical cases in a very dense hollow sphere, the diameter of 
this sphere being about half that of the entire oogonium. 
Very frequently, however, instead of forming a sphere, the 
aggregation of nuclei may take the form of an irregular 
crescent either in the upper or lower half of the oogonium. 
In less frequent cases several masses of nuclei are formed 
instead of a single one. 
The cytoplasm of the oogonium around this mass of nuclei 
has entirely changed its appearance. Instead of being charged 
with strongly staining substances it has become extremely 
tenuous and loosely spongy in its texture (Figs. 14, 15), 
so that the nuclei stand out much more sharply differentiated 
than before. The nuclei in this central mass are embedded 
in a cytoplasm which is essentially similar to that in the 
periphery of the oogonium. There has been an actual dis¬ 
appearance from the protoplasm of a considerable amount 
of densely staining substance at this stage, but its fate is by 
no means certain. The oogonium has also increased some¬ 
what in size and its wall has become somewhat thicker; but 
whether these ordinary phenomena of growth are sufficient 
to account for the change in the density of the protoplasm 
must be left undetermined. After the oogonial nuclei have 
become aggregated in the centre of the oogonium the basal 
wall of the conjugation-tube breaks down (Figs. 15, 150), 
and the antheridial nuclei are admitted to the oogonium 
where they pass at once to the central mass of egg-nuclei 
and become mingled with them. Fig. 14 shows that there 
is no special differentiation of the protoplasm on either side 
of the basal wall of the conjugation-tube either before or 
at the time of its disintegration. There is no formation 
