1914] Digestion and Digestive Epithelium in Insects 319 
These I believe are the absorptive cells and correspond to the 
cells marked ab. in Fig. 7. Of course this larva has had no plant 
food yet, but we must suppose that these cells contain some- 
thing besides food products. As a matter of fact they are 
much leaner than in older larve except for the basal part 
containing the nucleus, being scarcely distinguishable between 
the secretory cells. These are beginning to form the secretion, 
though none of them has any protruding droplets. Larve 
sectioned twenty-four hours after beginning to feed, show some 
food in the alimentary canal, and some of the secretory cells 
are giving off small drops of fluid. As the larva grows these 
droplets increase in size until they appear as in Fig. 10 or 11. 
The contents of the absorptive cells is homogeneous, and takes 
the Orange G stain precisely as does the contents of the silk 
glands, which indicates that it is a product of digestion, as the 
liquid silk is merely this same product somewhat transformed. 
The changes in the cells during molting I have already des- 
cribed. 
As a killing fluid I found nothing superior to Carnoy’s 
fluid, which is a mixture of 6 parts of absolute alcohol, 3 parts 
of glacial acetic acid, and one part of chloroform. For general 
insect work it is entirely satisfactory. The chloroform very 
quickly dissolves any wax or grease, and allows the insect to 
sink. It acts quickly, and is very simple to use. The speci- 
mens are merely dropped into it, left for a couple of hours, and 
transferred to alcohol, first 70% and then 85%. There was 
some distortion and occasionally shrinkage, but I am inclined 
to believe that this latter was due to some fault in the subse- 
quent treatment. Tower’s, Gilson’s, hot water, etc., gave no bet- 
ter results, and are not so easily handled. I tried injecting a 
number of silkworms with the killing fluid, just after dropping 
them into it, but could see no difference between these and those 
not injected. The specimens were run through alcohol and im- 
bedded in the ordinary way. Sections were cut as thin as 
possible. I tried a variety of stains and found combinations 
either of Ehrlich’s acid hematoxylin or iron hematoxylin with 
Orange G to be the most satisfactory. Occasional preparations 
stained with carmine and Orange G brought out points not 
observable in others. The Orange G is a very desirable second- 
ary stain, as it stains tissues which otherwise would remain 
almost colorless. 
