356 Harper.—Sexual Reproduction in Pyronema 
fusing pairs consist in all cases of male and female pronuclei 
from the antheridium and the oogonium respectively. It is 
also impossible, as I have just mentioned, to determine with 
certainty the behaviour of the nuclei in the centre of these 
masses ; still, the large number of cases of fusion that can be 
observed leaves little doubt that the rule of fusion in pairs 
holds for practically the whole mass. 
It will be observed that these fusions are not necessarily 
simultaneous, some pairs being completely united while other 
nuclei are not yet arranged in pairs. In the oogonium from 
which Fig. 15 is taken, it is to be seen in other sections that 
a few nuclei have already begun to wander out into the 
young ascogenous hyphae, while the ascogenous hyphae of 
the oogonium, from which Fig. 15 a is taken, contain as yet 
no nuclei. 
It is not to be assumed from my statement as to the 
density of this mass of nuclei in the oogonium that anything 
in the nature of a general fusion of the nuclei into a single 
nucleus occurs. It is perfectly certain, as seen from my 
Figs. 15 and 15 a, that throughout these masses individual 
nuclei of typical appearance can be made out with perfect 
certainty and at a slightly later stage, when the nuclei scatter 
after fusion, it is still further apparent that the mass consisted 
of perfectly distinct nuclei (PI. XX, Figs. 17 a, b). 
This aggregation of the nuclei at the time of fertilization 
seems to be simply a provision for the pairing of the male 
and female nuclei with the greatest certainty and dispatch. 
If the sexual nuclei are attracted to each other chemotactic- 
ally, it is easy to see that unnecessary migrations of the male 
nuclei after attempts at multiple fusions are avoided by 
having the female nuclei in close proximity with each other, 
and in as small an area as possible. If the female nuclei 
were irregularly distributed it would be possible that isolated 
unpaired male and female nuclei might remain at opposite 
sides o*f the oogonium outside the spheres of their mutual 
chemotactic influence. Pyronema thus affords an interesting 
example of the fusion of multinucleated sexual cells and the 
