IO6 THE FROG 



of thin cork in such a manner as to stretch the mesentery, 

 and place the entire specimen in the fixing fluid; cut the 

 mesentery into small pieces and stain in the manner directed 

 for the bladder. Isolated fibers may be prepared by macer- 

 ating pieces of the stomach, intestine or bladder for two or 

 three days in 20 per cent, nitric acid, teasing on a slide or 

 shaking in a vial containing water, and staining with picro- 

 carmine. 



9. Connective tissue. As directed in the text, material 

 may be taken from a frog hardened in formalin, and studied 

 without staining, but there are some advantages in using 

 permanent stained preparations for comparison. The thin 

 septa which fasten the skin of the frog to the body consist 

 for the most part of white fibrous connective tissue; cut out 

 pieces of this from a fresh frog, spread on a slide and fix 

 with corrosive sublimate (avoid the use of fixing fluids con- 

 taining acetic acid, since this tends to dissolve the white 

 fibers leaving the yellow elastic fibers exposed). Stain with 

 Mal'lory's connective tissue stain (see Guyer, Animal Mi- 

 crology). 



10. Cross-sections of the intestine.- -Using a freshly- 

 killed frog that has not taken food for several days, cut out 

 pieces of the upper part of the small intestine about 15 

 millimeters long with a narrow strip of the mesentery at- 

 tached. Fix 12 to 24 hours in Zenker's fluid. Cut sections 

 6/x to 8^. thick. Stain on the slide with Delafield's hsema- 

 toxylin and counterstain with orange G or Congo red; or 

 stain in bulk with borax-carmine or paracarmine and coun- 

 terstain on the slide with the Lyons-blue picric-acid mixture. 

 The latter method gives better differentiation of the goblet 

 cells; in other respects the former method is more satis- 

 factory. 



