THE FROG ie) 
6. Cross-sections of the intestine. Using a freshly-killed frog 
that has not taken food for several days, cut out short pieces of 
the intestine (about 1 centimeter long) with a narrow strip of 
the mesentery attached. Fix 12 to 16 hours in Zenker’s fluid. 
Stain on the slide with Delafield’s hemotoxylin and counterstain 
with Orange G; or stain in bulk with borax carmine or paracar- 
mine and counterstain with the Lyons-blue picric-acid mixture. 
The latter method gives better differentiation of the goblet cells. 
Cut sections 6 u to 8 p thick. 
7. Cross-sections of the stomach. Using a freshly-killed 
young freg with stomach in the resting condition, cut out the 
entire stomach and fix 24 hours in Zenker’s fluid (corrosive-acetic 
does not penetrate well enough to fix the nuclei of the mucosa 
satisfactorily). After passing the object to 96 per cent. alcohol, 
trim off the pyloric portion and the extreme cardiac end. Stain 
by either method recommended for the intestine. Sections should 
be cut with a thickness not exceeding 8 p. 
8. Cross-sections of the kidney. Remove the kidney from a 
fresh frog, cutting around the organ so as to include the peri- 
toneum and adjacent portion of the ureter (care must be taken 
not to tear off the peritoneum from the dorsal surface of the 
kidney). Fix about 12 to 16 hours in Zenker’s fluid. Discard 
the anterior third and the extreme posterior end, and cut sections 
8p in thickness. Stain on the slide with Delafield’s hamotoxylin 
and Orange G. 
9. Sections of the thyroid gland. Since the gland is ex- 
tremely small, a piece of cartilage may be left attached to aid in 
handling it. Fix for a few hours in Zenker’s, cut thin sections 
and stain with Delafield’s hemotoxylin and Orange G. 
10. Cross-sections of the integument. Select a frog whose 
skin is bright-colored, and kill it by pithing, not with anesthetics 
since the latter may cause too profuse secretion from the mucus 
and poison glands. From the dorsal surface cut out a strip 
that includes both lightly-pigmented and deeply-pigmented por- 
tions as well as some poison glands (the latter are especially 
numerous in the dorso-lateral line). Fix for a few hours in 
Zenker’s; stain on the slide with Delafield’s hemotoxylin and 
Orange G, or stain in bulk with borax-carmine and counterstain 
