90 THE FROG 



the dry gelatin in distilled water until it is swollen and soft 

 (about 1 8 hours), then remove to a porcelain evaporating 

 dish and melt at a temperature of about 45 C. The in- 

 jection causes the tongue to protrude as in the act of seizing 

 food; the pressure should be kept up until the gelatin 

 hardens, when the tongue will be kept permanently in this 

 position. The frog should then be hardened in formalin and 

 exhibited in formalin or in alcohol in a glass jar. 



In the study of the digestive system the bile duct and its 

 relation to the bile sac, pancreas and intestine require special 

 attention. Using a large frog, make a preparation showing 

 the organs represented in Fig. 42 of Holmes' Biology of the 

 Frog. The parts should be pinned out on a wax plate, such 

 as is used for dissecting trays, which may be made as fol- 

 lows : Melt some beeswax, soft commercial paraffin, or a mix- 

 ture of these two substances and stir in sufficient lampblack 

 to color it an intense black; pour out into a shallow tray 

 enough of the mixture to form a layer about 8 millimeters 

 thick, and allow it to cool. The plate may then be cut to the 

 required dimensions. The preparation should be exhibited 

 in a rectangular glass jar containing alcohol rather than 

 formalin, since the latter will attack the pins. For fastening 

 the preparation to the wax plate, porcupine quills have some 

 advantage over metallic pins. 



B. The Urogenital System.- -Using fresh material, 

 make preparations of the reproductive systems of a male 

 and a female frog by exposing the organs so far as possible 

 in situ. The bladder* should be injected with gelatin. In 

 the case of the female, the ovary of one side may be re- 

 moved in order to display the other organs to better ad- 

 vantage. Since the oviduct readily absorbs water causing 



