PHYSIOLOGY OF DIGESTION IN BLATTIDAE aI 
olive oil, for the pure oil consists mostly of the glyceride of oleic 
acid, and oleic acid is the only one of the four common fatty 
acids of ordinary fats which is blackened well with osmic acid, 
this being due to its unsaturated molecular structure. In work 
with tissues containing blackened fat it seemed inadvisable to 
stain with any haematoxylin stains, for these stain so darkly and 
tend to obscure appearances of the darkly stained fat. As a gen- 
eral stain acid fuchsin was used successfully; it stains nuclear 
and cytoplasmic structures sufficiently distinctly. For obtaining 
the best histological details Perenyi’s fluid was much used, and 
also a mixture of 5 per cent formol and 50 per cent aleohol. ‘The 
best combination of stains for accurate. definition of structures 
was acid fuchsin, followed by Ehrlich’s haematoxylin. In 
working with the very compact and hard tissues of the gizzard, 
Flemming’s killing fluid was very satisfactory when it penetrated 
well, which was not always the case. Perenyi’s fluid was also 
satisfactory. In order to bring out the muscle structures of the 
gizzard, careful staining with iron haematoxylin and picro- 
fuchsin is necessary. When the animals had eaten fat stained 
with Sudan III or Nile blue sulphate, sections were made in a 
freezing microtome. 
Most of the tissues contain so much water that extra care is 
necessary in running them through a series of reagents. In 
running ordinary tissues from absolute alcohol through xylol to 
melted paraffin, it is very desirable to interpose between the 
alcohol and xylol two appropriate mixtures of these reagents. 
In running from xylol to melted paraffin, it is necessary to inter- 
pose a cold and a warm mixture of the two. Unless this is 
done, the nuclei and cytoplasm shrivel and the fat globules be- 
come distorted and liable to fade rapidly. 
The fat used as food in the experiments was pure olive oil. 
The animals refused to take pieces of solid fat, but would take 
bread soaked in olive oil. But the ideal food seemed to be a 
paste of pulverized sugar and olive oil. This was taken greedily, 
even when stained brightly with Sudan III, Nile blue sulphate, 
or litmus powder. The organs chiefly concerned in recognizing 
the food semed to be the maxillary and labial palpi, not the eyes 
