Cytology of Pythium ultimum, n, sp. 299 
When karyokinesis begins the chromatic mass gradually 
breaks up, the chromatin passes out on to the linin threads, 
and a distinct nucleolus remains behind. As the spirem stage 
is reached, the network of linin bearing the chromatin granules 
becomes very evident. The nucleolus sometimes persists until 
the spindle has been fully formed, and even until the meta- 
phase condition is readied. The nuclear membrane can be 
recognized until the nucleus is in metaphase, but at later 
stages it is not possible to trace it definitely. The achromatic 
figure appears to arise entirely within the nucleus, and of its 
threads those which serve to connect the daughter-chromo- 
somes can be readily recognized in the last stages of division. 
The number of chromosomes is certainly as many as six, 
probably more. It is difficult to get definite results on this 
point. If the stain is too deep there is a danger of counting 
two adjacent chromosomes as one ; if not deep enough the 
chromosomes are apt to be overlooked. The chromosomes 
are, in fact, so small that deep differential staining and the 
best objectives are necessary for their proper resolution. The 
nuclei, at all stages in the life history, agree in general 
appearance when seen in division, the impression being 
created that there is no variation in the number of the 
chromosomes. 
Observations of Taxonomic Interest. 
At an early stage of the investigation it became apparent 
that the Fungus was either a species hitherto undescribed or 
described so inadequately as to be difficult of recognition. 
The knowledge which we have gained of its characters leaves 
no doubt as to the affinities of the species. It resembles 
Pythium vexans most closely, a species, as Fischer ( J 92) points 
out, incompletely investigated and inadequately described by 
De Bary (’81). 
De Bary’s figures of P. vexans , however, show conclusively 
that our species is a distinct one. In fact, if we are to unite 
it with P. vexans we must ascribe to De Bary a carelessness 
and want of insight altogether foreign to that brilliant and 
