66 THE FROG 
and cut it across where it is joined by the small intestine. Re- 
move the large intestine, together with the cloaca, bladder, 
kidneys, gonads, fat bodies and the ducts which enter the cloaca. 
Spread out the preparation on a wax plate to be used in mounting 
it (see directions for the digestive system), and clean up the peri- 
toneum and blood vessels about the ureters. Trace the ducts 
to their union with the cloaca, slit the cloaca open along a ventro- 
lateral side and find the openings of the ducts and the bladder; 
insert bristles into the openings, using differently colored bristles 
to distinguish the various kinds of openings. Insert bristles into 
the openings of the oviducts at their anterior ends. After arrang- 
ing the various parts in suitable positions so that they may be 
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 urogenital 
system are peculiarly necessary. 
C. The Circulatory System.—1. The heart of the average- 
sized specimen of Rana pipiens is rather small for satisfactory 
dissection, consequently this work should be supplemented 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 structures of the dorsal surface should be shown, 
since these give most difficulty in the dissection of a small frog; 
the sinus venosus, vene cave 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 in- 
ternal structures represented in figure 72 of Holmes’ Biology of 
the Frog. 
2. The arteries should be injected with some colored substance 
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 
