182 PREPARATION AND MOUNTING 



ject without so much trouble. With a half-inch power the blood 

 may now be seen to flow very distinctly. The frog may be used 

 for hours if care is taken to prevent the web from becoming dry, 

 by wetting it with a little water from time to time. The piece 

 of wood or cork upon which the frog is laid is often made to give 

 place to the " frog-plates," supplied by opticians. These are made 

 of brass, somewhat resembling the piece of wood above recom- 

 mended, but each maker's pattern differs according to his own 

 taste. 



The tongue of the frog is also sometimes used for the purpose 

 of showing the circulation of the blood, which is done in the fol- 

 lowing manner : The body is wrapped with the calico, and made 

 fast to the plate as before, only the mouth of the frog is brought 

 to the opening. The tongue is then gently drawn out of the 

 mouth and pinned down over the aperture, when the circulation 

 will be well show^n. But, as Dr. Carpenter observes, the cruelty 

 of this mode of treatment is so repulsive that it is unjustifiable. 



Tadpoles of the frog (which, of course, are only obtainable in 

 their season) are good subjects for showing the circulation of the 

 blood. They are best suited for the microscope when about one 

 inch long. The tadpoles of the newt and toad also are equally 

 suitable. They may be placed in a very shallow glass trough 

 with a little water, and a narrow band of linen bound lightly 

 round in some part not required for examination, to keep them 

 from moving ; or they may be laid upon a glass plate with a drop 

 or two of water, and a thin glass covering lightly bound upon it. 

 Dr. Carpenter, however, places them first in cold water, gradually 

 adding warm until the whole becomes about 100, when the tad- 

 pole becomes rigid, whilst the circulation is still maintained. I 

 have not, however, found this necessary, the thin glass accom- 

 plishing all that is desired. The tail is generally the most trans- 

 parent, and shows the circulation best ; but in some of the newt 

 larvae the blood may be traced down to the very extremities if 

 they are not too old. Mr. Whitney places the tadpole upon its 

 back, by which means the heart and other internal arrangements 

 may be seen. 



Amongst fishes also may be found subjects for the same pur- 

 pose, but they seldom furnish as good examples as those before 



