PHYSIOLOGY OF DIGESTION IN BLATTIDAE 399 
tained leucocytes. The deposit in the tracheae is always much 
thicker than blood; it may be that some of the water diffuses 
out of the blood in the tubes. Perhaps the leucocytes ingest 
some of the unnatural substance present. The following experi- 
ment was performed to determine whether leucocytes might 
enter tracheal tubes which contained en unnatura’ substance. 
A normal animal was immersed in olive oil which was stained 
red. The animal was dissected, and the tracheal trunks were 
found to contain much red fat. <A piece of one, filled with fat, 
was removed and inserted into the body cavity of another 
normal animal. ‘This animal was dissected three days later; the 
red trachea was removed, preserved in Flemming and sectioned. 
The tube was found to contain many leucocytes which had en- 
tered in response to the unnatural substance, and were ingesting 
it, as evidenced by the globules of ingested fat in their cytoplasm. 
The result of the experiment suggests that leucocytes may enter 
any rips which occur in the tracheae and may ingest any foreign 
material there, whether fat, thickened blood plasma or other sub- 
stances. Such a theory is incapable of proof. It is almost 
certain in any case that the chyme is not to be considered as a 
really normal content. 
My investigations have shown that three of Petrunkevitch’s 
four striking appearances represent usual conditions, his slides 
agreeing closely with mine. ‘Therefore Schliiter’s absolute and 
eritical denial of his observations is unjustified. As to the 
interpretation of these four observations, I am convinced that 
they represent four distinct processes, «wo of them normal, 
and not four stages in one process, as Petrunkevitch thought. 
Blanchard’s ‘peritracheal circulation’ theory should be consid- 
ered here. In order to test whether substances really can pass 
from the blood into the tracheae, as Blanchard described, vari- 
ous substances were injected into the body cavity and later 
searched for in the tracheae. Such injections had little or no 
toxic effect. The best substance to use is phenol-sulphone- 
phthalein, which passes through the body dissolved in the body 
fluids. The presence of this stain could not be detected in the 
tubes at any of several periods after injection, though the stain 
