PHYSIOLOGY OF DIGESTION IN BLATTIDAE 397 
in the region of the cut ends and for a distance beyond. This 
surely did not represent a normal process, but resulted from 
some factor which had been acting through the experiment. It 
seems certain that the negative pressure in the tracheae, as men- 
tioned above, had exerted a sucking action at points where open- 
ings were made. Such openings were made when the crop was 
slit longitudinally, and as a result the substance at the cut end 
(in this case olive oil) had been drawn in. This experiment was 
repeated on other animals and showed conclusively that the fat 
which Petrunkevitch and I had found in minute tracheae in our 
preparations had been sucked in by the tubes through their cut 
ends during the dissection, and was therefore no normal content, 
but an artifact. We had easily overlooked it because such small 
structures were involved and because we had used colorless fats. 
It was interesting to explain why Petrunkevitch found fat in the 
tubes only at considerable intervals after feeding, while I found 
it at many stages. ‘This is simply due to the fact that Petrunke- 
vitch fed solid fat which required some time to be liquefied and 
changed to fatty acid and thus made capable of being sucked into 
tubes. In my work, liquid fat or fatty acid was always present 
in the crop after feeding, and it was thus always possible that it 
be drawn into the tracheal branches by suction, no matter at 
what period after feeding the cut was made. 
The above experiments demonstrate that fat in tracheal tubes 
is abnormal; its presence may be accounted for in three ways: 
1) Suction of fat through minute tracheae which are cut when 
the crop is opened in dissection; this suction may occur centrif- 
ugally or centripetally; 2) suction through large tracheal trunks 
and through spiracles; 3) capillarity. These three factors seem 
sufficient to explain the pictures of the several investigators who 
have so long discussed the problem; they indicate that fat is 
never normally present in the tracheal tubes. 
The other three appearances which Petrunkevitch described 
and figured must now be explained, and, firstly, the globules of 
osmicated fat in the tracheal end cells. I have verified this in 
various preparations. These cells resemble in structure the epi- 
thelial cells, so it is natural that they absorb soluble food products 
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 25, NO. 2 
