April 6, 1888,] 



SCIENTIFIC NEWS. 



TADPOLES. 



WE said something lately (Scientific News, page 269) 

 about a fowl's egg and its development, and now 

 give for comparison a sketch of another type. Workers at 

 natural history may be interested in a few plain hints as 

 to what is to be made out in the eggs of a frog and the 

 tadpoles which they produce. 



The eggs of the frog are laid in March, and among the 

 first signs of spring are the bunches of frog-spawn 

 floating in the ditches. Every such bunch consists of 

 some hundreds of black eggs, each surrounded by its 



the swelling of the albuminous covering, no egg inter- 

 cepts too much the light ; and water containing dissolved 

 air can permeate the mass. Secondly, the elastic and 

 slippery investment renders the egg too smooth to be 

 picked up by birds and predatory insects, which would 

 gladly devour the floating spawn. Try to grasp a mass 

 for yourself. As your fingers vainly close upon the 

 slimy stuff, you learn how effectually the eggs are pro- 

 tected from bills and jaws. Lastly, the albuminous 

 covering is a defence against parasitic moulds. There 

 are many aquatic moulds which fasten readily upon 

 animal matter and destroy it. They cannot touch the 



Tadpole with long tail, hind-limbs well developed, fore-limbs not externally visible, 

 (ventricle) ; G, internal gill ; St, stomach ; Lv, liver ; Lg, lung. 



M. MOUTH ; H. HEART 



own envelope of transparent slime. The black spot is 

 the essential part of the egg, and the only part present 

 in the ovary of the female. The outer envelope is de- 

 posited whilst the egg is travelling down the oviduct, 

 and is therefore similar in the manner of its formation to 

 the white of the fowl's egg. At the moment when the 

 egg gains the water, this envelope is scanty, but it soon 

 swells by the absorption of water, and comes to greatly 

 exceed the egg itself in bulk. What is the purpose of 

 this glairy envelope ? First of all, it is useful to space 

 the eggs, which need, above all things, light and air. If 

 they were packed closely, the inner ones would probably 

 never develop at all ; but being gently pushed apart by 



frog's egg, protected by its smooth and watery coat, for 

 this secretion contains some preservative of unknown 

 kind which absolutely secures it from all the microscopic 

 organisms which engender mould and putrefaction. The 

 experiment has been tried of gently removing the albu- 

 minous covering. All the eggs so treated were speedily 

 attacked by the aquatic moulds and perished. The 

 envelope of the frog's egg is not nutritive, like the white 

 of the fowl's egg. It is left untouched after the tadpoles 

 have hatched out, and they only find it worth eating 

 when it has become overgrown with green cells, which 

 usually happens after a few weeks. 



What can the naturalist untrained to th'^ methods ot 



