The spurious parasites which are of most practical importance are 
the numerous objects, mostly plant material, which occur in feces and 
which resemble parasite eggs. To differentiate such material several 
tests may be applied. Such spurious parasites are frequently very 
different in size from the eggs which they resemble, and an accurate 
measurement will often show that they are not the eggs they are 
supposed to be. Plant material as a rule is more dense, more often 
colored, and shows less cell structure internally than do parasite eggs. 
Cestode eggs may be definitely determined by the presence of the 
6 hooks of the onchosphere. Fluke eggs usually show an operculum 
or lid at one end. Nematode eggs are very variable in size and shape, 
Fig. 74. Two views of the diatom Pinnularia viridis. Enlarged. From Campbell, 1902. 
but they usually contain an embryo or else cells in process of division, 
and the shell is definitely delimited from the egg content to an extent 
which is not true of the external coat and the content of a plant spore. 
In a general way it is true here as elsewhere that other things may 
look like the thing you are in search of, but the thing itself is un¬ 
mistakable when you find it. A parasite egg is evidently a parasite 
egg; it doesn’t merely look like a parasite egg. Doubtful eggs are 
usually to be regarded as not eggs. As a final test of the matter one 
may sometimes use chemical reagents to differentiate plant material 
from animal material. The addition of tincture of iodin to a slide will 
color starches blue. By adding sulphuric acid and iodin cellulose sub- 
