APPENDIX 153 



(formula for injection mass on page 159). No ligature will 

 be needed since the valves of the truncus prevent the back- 

 ward flow of the mass. During the injection of the arteries 

 the blood has been forced into the veins, and because of 

 this it is possible to follow the veins fairly well without in- 

 jecting them. After the injection is complete, the blood 

 and excess injection mass should be w r ashed off, and the 

 frogs preserved in formalin. 



For the study of the histology of the frog examine fresh 

 tissues in normal salt solution. For sections of the spinal 

 cord fix in 10 per cent formalin, embed in paraffin and make 

 rather thick sections. Borax carmine or hematoxylin are 

 good stains to show the general form and something of the 

 cells. For a sharp distinction of the white and gray sub- 

 stances Weigert's hematoxylin method for medullated 

 nerves gives beautiful results. (See directions in books on 

 technic. The mordant of copper acetate may be used after 

 the sections are on the slide.) For sections of the stomach 

 cut the organ into two parts, rinse in salt solution and fix 

 tw T enty-four hours in Zenker's fluid. Hematoxylin and 

 Congo-red are good stains to differentiate the tissues. 



In the spring get spawn from ponds and place small masses 

 in jars for the study of the development. Water plants in 

 the jars will help to aerate the water. If the water at any 

 time appears turbid, change. The tadpoles may be kept 

 for some time after hatching if the aquaria are kept supplied 

 with algae or other water plants. 



Preparation and Mounting of Slides. 



To get the best results in mounting small objects or 

 sections, the animal or tissue must be properly prepared. 

 It must be killed quickly so that the constituents of the 



