of the Nuclei in Peronosperct parasitica. 13 1 
and mounted in glycerine jelly. None of these preparations, 
however, were found to exhibit the structure of the nucleus 
satisfactorily, as the protoplasm itself was stained too deeply, 
and sections could not be obtained thin enough to allow the 
nucleus to be distinctly seen. 
The most successful preparations have been made by means 
of a Cambridge ribbon-section-cutting microtome. It may be 
useful here to give a few details as to the method employed. 
The fresh infected tissues of the Shepherd’s Purse were cut 
up into small pieces, and placed at once either in absolute 
alcohol or chromic acid solution. They were kept here until 
thoroughly penetrated, and were then prepared for embedding 
in paraffin wax. The method of preparation differs slightly 
from that required for animal tissues. The chromic acid 
specimens were thoroughly washed in 70 per cent, of alcohol, 
then transferred to methylated alcohol, and finally to absolute 
alcohol. Tissues prepared in absolute alcohol do not, of 
course, require these intermediate washings. The pieces of 
tissue may then be stained en bloc , or the separate sections 
may be stained, when cut, on the slide. The latter method 
was found to be preferable. 
For staining en bloc the pieces of tissue were transferred to 
the staining fluid, a strong solution of haematoxylin in 
Kleinenberg’s solution. They were left in this for a few days, 
and were then successively washed in 70 per cent., 90 per cent, 
and ico per cent, alcohol. They were left in the absolute 
alcohol for twenty-four hours. The tissues should be 
thoroughly dehydrated, otherwise the greatest difficulty will 
be found in embedding them satisfactorily. After being 
thoroughly dehydrated in alcohol, they were transferred to 
turpentine for about forty-eight hours, and were then placed 
in soft melted paraffin wax for about twenty-four hours, and 
were finally transferred into hard melted paraffin wax for 
about two days. I have constantly used a wax melting at 
59 0 C. The softer wax should have a melting-point of about 
49 0 to 50° C. The pieces of tissue were then embedded in 
small square blocks of paraffin, and very thin sections cut by 
