Cytology of Pythium ultimum , n. sp. 289 
four exceptions, as already stated, are based upon preparations 
made from the pure cultures, fixed with Flemming’s stronger 
solution for five or more hours, and stained with gentian 
violet. 
The sexual organs and oospores. Figs. 6 to 45 represent 
consecutive stages in the development of the sexual organs, 
in fertilization, and in the maturation and germination of the 
oospores. The sequence may not be absolutely correct, but 
such errors as may unavoidably have crept in must be of 
small moment. Comparisons should be made with the 
corresponding figures representing the condition in the living 
material. Relative age may frequently be determined fairly 
well by the size of the organ, the distribution of the contents, 
and the thickness of the cell-wall. 
At first it was intended to make careful countings of the 
nuclei in the different stages. This idea was soon abandoned, 
for even with sections 5 \ a thick the number of available 
sections in a series was generally but three, and so short a 
series is difficult to follow accurately because of the rapid 
change in the appearance of the sections. Indeed one has 
not only to find suitable sections, but sections so placed that 
there is no danger of passing from one oogonium to another. 
Moreover it was found that the median section of three was 
quite sufficient to give one a clear idea of the condition of the 
remaining two, and so of the whole. It is necessary perhaps 
to add that, nevertheless, a series of sections was generally 
examined before a median one was adopted for detailed study. 
The reader who has the drawings of the median sections 
before him has therefore a full opportunity of criticizing the 
views founded upon their examination. Had drawings of 
the adjacent sections been added in each case no additional 
feature of importance would be noticeable. 
Fig. 6 represents a half-grown oogonium. The nuclei are 
small. The number is in this case exceptionally large, but 
the remaining two sections only contained four additional 
nuclei. The number of nuclei in a young oogonium is very 
variable, ranging from twelve to twenty-five or thirty, and the 
u 
