84 Sir Everard Home on the changes the ovum of the frog 
resembles that of the frog. Upon reflecting that the frog's 
spawn is exposed to the scorching effect of the sun, and in 
places where there is no shelter, this nigrum pigmentum 
may be given to the eggs as a defence for the young during 
its growth, which cannot be required in those of the aquatic 
salamander, since they are separately inclosed within the 
twisted leaves of water plants, and screened from the full 
force of the sun's rays. The plant whose leaves the aquatic 
salamander most generally selects to lay its eggs upon is the 
Polygonum persicare. 
Explanation of the Plates. 
Plate V. 
Fig. 1. A female frog laid open, just ready to shed her 
spawn ; natural size. 
Fig. 2. The ovaria and oviducts ; natural size. 
Fig. 3. Ova of different sizes taken from the ovarium ; 
magnified five diameters. 
Fig. 4. Ova from the upper part of the oviduct ; magnified 
five diameters. 
Fig. 5 . Ova from the dilated portion of oviduct ; natural 
size. A B. Two ova that had been a few minutes in water, 
to show the expansion of jelly ; magnified five diameters. 
Fig. 6 . An ovum from which the jelly is removed ; mag- 
nified ten diameters. 
Fig. 7 . The same ovum opened, to show that the contents 
are fluid ; when they are allowed to coagulate and dried upon 
glass, ramifications are formed as in coagulating blood ; 
magnified ten diameters. 
