396 ELDON W..SANFORD 
stance in their tracheae, nor did a series of animals which were 
treated similarly but for the fact that their crops were preserved 
in Flemming and sectioned; microscopic examination of sec- 
tions of this material revealed no osmicated fat in the tracheae. 
The experiments had comprised all usual digestive pe iods 
and had included so many animals that individual variation and 
other complicating factors were negligible. The result was con- 
sidered final and definite; it was certain that fat did not nor- 
mally enter the tracheal tubes, as I had previously thought. I 
realized that all the fat I had seen in tracheae in earlier prepa- 
rations must have come from an abnormal source. 
I next attempted to analyze why my earlier experiments, like 
Petrunkevitch’s, showed fat in the tubes, while more careful 
controls did not. From previous observations with colored fat I 
knew that oil might enter the cut ends of large tracheal trunks, 
but obviously this did not explain why Petrunkevitch and I had 
found fat present in only certain small branches and groups of 
branches. Another factor had obviously been acting. In at- 
tempting to explain this I performed the following experiment: 
An animal which had eaten some red paste two days before 
was pinned, while still active, dorsal side down on a dissecting 
pan. The spiracles were then thickly smeared with celloidin 
solution. This soon dried and formed a complete film over all 
the spiracles, thus preventing the entrance of any substance 
into the tracheal trunks within. The passage of air was pre- 
vented, but the still living animal performed to some extent 
breathing movements. ‘The result of these movements is suc- 
tion, but nothing could be sucked in, so the suction existed in 
the tubes without any possibility of anything being sucked in. 
The animal’s body wall was carefully slit and pinned to the side 
without dirturbing the tracheae. Now the wall of the crop was 
slit longitudinally. This necessarily involved cutting many of 
the tracheal branches of various sizes which ramify over the 
crop. As soon as the animal was surely dead, the crop was re- 
moved and its outer surface examined microscopically. The 
small tracheae which had been unavoidably cut were especially 
observed, and it was found that some of them contained red fat 
