TOADS AND PROGS. 61 



The Aquarium. — Before collecting the eggs prepare a 

 hatching-pond. In a milk-pan or granite-ware or porcelain 

 basin place some sand or scrapings from a pond, mingling 

 therewith stones laid in such manner that at one side of the 

 basin they come to the top of the water and at the opposite 

 side three or four inches below its surface. It is advantageous 

 that some of the stones have green algae growing on them, but 

 whether they have or not plant a few weeds or mosses 

 that naturally live under water and keep green in it in the 

 basin. See the note on the Aquarium, page 26. Do not put 

 in too many eggs; two or three dozens are plenty, indeed that 

 many tadpoles would greatly overstock a milk-pan pond. The 

 number it will support depends on the food supply ; in any 

 case a dozen large tadpoles would be rather too many 



The Gelatinous Covering of the Eggs. — The eggs when laid 

 vary in size according to the species from a tenth to a fifth of 

 an inch, but the thin gelatinous covering absorbs water and 

 swells up to the size of large peas or small marbles. The jelly 

 keeps them afloat, while their slipperiness probably protects 

 them from seizure by fish and birds. The embryo is on the 

 dark side ; the white part is a store of food. The colder the 

 water the more slowly the embryo develops ; water that is too 

 warm would quickly kill them. The tadpoles may begin to 

 appear in two or three days after you stock the basin, or you may 

 have to wait two weeks for them to hatch. Experiments. — 

 (1) Try to pick up a frog's egg out of water with a pair of 

 forceps. Infer the difficulty a bird would experience in 

 attempting to feed upon such eggs. (2) Put equal weights 

 of snow or ice on blotting-paper, covering one with a piece of 

 white cloth and the other with black cloth of similar stuff. 

 Expose the preparation to direct sunlight, and observe how 

 much more quickly the substance covered by the black cloth 

 melts than the other. Infer the use of the black layer over 

 the embryo in facilitating hatching. 



