Cytology of Pythium ultimum , n. sp. 309 
on germination. No zoospores have been observed. The 
species is consequently new, and ranks as the most highly 
developed of the genus. The generally accepted definition 
and affinities of the genus require considerable modification. 
The conidia germinate at once in cabbage-water, but remain 
at rest in distilled water. The behaviour of the oospores 
remains to be further investigated, but it is certain that they 
will germinate as soon as ripe, or after a rest of seven 
months. 
The mycelium, conidia, oogonia, and antheridia are multi- 
nucleate, the oosphere and the ripe oospore uninucleate, and 
the young oospore binucleate. 
The nuclei multiply in the mycelium and sexual organs by 
indirect division. No nuclear divisions have been seen in 
the conidia except during germination. No nuclear fusions 
take place other than those of the male and female nuclei in 
fertilization. The number of chromosomes is considerable, 
certainly six or more. 
The oogonium, as it is formed, receives twelve or more 
nuclei, the antheridium three or more. These invariably 
divide once, so that the number of nuclei, although already 
greatly in excess of the requirements of the organs, is doubled. 
The supernumerary nuclei in the oogonium pass into the 
periplasm, one only remains behind and occupies the centre 
of the egg. No similar differentiation takes place in the 
antheridium. 
The fertilization-tube penetrates the wall of the oogonium, 
passes through the periplasm, and penetrates deeply into the 
egg. One male nucleus passes down the tube and enters 
the egg. The oosphere clothes itself with a delicate cell-wall 
and proceeds to digest and absorb the periplasm, increasing 
in size during the process. 
The male and female nuclei do not fuse until a thick 
oospore-wall has been produced. As the oospore ripens a 
reserve globule is formed in the centre of the oospore and 
the fusion-nucleus is forced to one side. No epispore is 
developed. 
