THE FROG 



1 6. Cross-sections of mammalian bone, dry .--With 

 a file or a fine saw, cut thin transverse sections of the fresh 

 femur of a cat, rabbit, pig or other small mammal. Re- 

 move the marrow and leave the material in water until 

 thoroughly macerated. This may require weeks or months, 

 depending on the size of the piece of bone; when dry the 

 material should be clean and white. After drying the disc 

 of bone thoroughly, cement it to a plate of glass by means 

 of Canada balsam, and grind it to the required thinness on 

 a clean hone. Both sides of the disc should be ground, 

 using first a coarse, then a fine hone. The section is not thin 

 enough until fine print can be distinguished through it. Re- 

 move the section by dissolving the balsam in xylol, remove 

 the xylol with absolute alcohol, wash the section thoroughly 

 in water, transfer it to absolute alcohol for ten minutes, then 

 to sulphuric ether for half an hour. Fasten the section be- 

 tween two slides by means of rubber bands and allow it to 

 dry thoroughly. Break the section into pieces of suitable 

 size for mounting. Mount dry, fastening the edges of the 

 cover glass with cement or thick balsam. 



17. Cross-sections of the spinal cord. In removing 

 the cord from the freshly-killed specimen, be careful to leave 

 the investing membranes intact. Fix the entire cord in 

 Zenker's fluid. After further hardening in alcohol, cut out 

 a piece about i centimeter long including the brachial en- 

 largement, and another slightly longer including the lumbar 

 enlargement; these regions are best to show the cell bodies 

 of nerve cells. Cut transverse sections 8p> to 10 p thick; 

 stain on the slide with Delafield's hsematoxylin and counter- 

 stain with orange G, Congo red, or eosin. 



1 8. Cross-sections of the brain. In removing the 



