9 2 THE FROG 



easily distinguished, fasten them to the wax with pins. In 

 the case of the female urogenital system the preparation 

 must be hardened and preserved in alcohol rather than in 

 formalin. 



It often happens that frogs purchased during the winter 

 are nearly all females, hence preparations of the male uro- 

 genital system are peculiarly necessary. 



C. The Circulatory System. i. The heart of the aver- 

 age-sized specimen of Rana pipiens is rather small for satis- 

 factory dissection, consequently this work should be supple- 

 mented by the examination of permanent preparations made 

 from the largest frogs obtainable. The preparations should 

 be kept in small cylinder jars or vials, containing alcohol or 

 formalin, in which they may be examined from all sides. 

 The preparations should include some entire hearts with 

 stumps of blood vessels attached. In particular the struc- 

 tures of the dorsal surface should be shown, since these give 

 most difficulty in the dissection of a small frog; the sinus 

 venosus, venae cavse and especially the pulmonary veins 

 should be intact and free from foreign tissue. In addition 

 there should be some dissections of the ventral side showing 

 the internal structures represented in Fig. 72 of Holmes' 

 Biology of the Frog. 



2. The arteries should be injected with some colored sub- 

 stance in order that they may be more easily distinguished. 

 For this purpose a starch injection mass is generally used, 

 since it flows freely when injected, and soon hardens so that 

 it will not readily escape from a vessel accidentally cut. It 

 should be made according to the following formula, in which 

 the parts are given by volume. 



