254 EEPORT 01' COMMISSIONER OF FISH AND FISHERIES. 



The eggs are extruded by the female and are fertilized by the male 

 as they pass out, very few failing to be impregnated. The process of 

 oviposition or laying continues through several days, and during this 

 period several hundred eggs may be deposited. The size of the ova 

 varies with the species, but averages about 1.75 millimeters (.07 inch) 

 in diameter. In passing down the oviduct the egg receives a thin 

 coating of albuminous material ; this rapidly swells when the egg enters 

 the water and forms the well-known gelatinous mass in which the frog 

 eggs are always found imbedded. The toad's eggs are laid in long 

 strings and are readily distinguishable. The salamander's eggs are also 

 X)laced in the water, but the gelatinous mass is somewhat firmer and 

 the eggs are slightly larger than the frog's, and they are usually 

 deposited somewhat earlier. 



The eggs begin development, under favorable circumstances, as soon 

 as fertilized, the rapidity depending mainly on the temperature of the 

 water; incubation is much retarded by cold, and some seasons many 

 eggs are killed by late frosts. At first the upper part of the eggs is 

 black and the lower white, but the rapid growth of the black embryo 

 makes the entire egg dark. The egg, which is at first spherical, soon 

 becomes ovoid. In from 4 to 30 days the tadpole is able to wriggle 

 out of its gelatinous envelope and shortly attaches itself to some plant 

 or other support by means of a sort of adhesive organ in front of the 

 mouth. At first the mouth and anus are closed, and food can only be 

 obtained by absorption, the first food consisting of the gelatinous egg- 

 envelope. In a short time the mouth and anus become functional, the 

 alimentary canal lengthens, and macerated animal and vegetable matter 

 becomes the food. The prevalent idea that the tadpole is exclusively 

 vegetarian, based on its anatomical structure, horny jaws, and long 

 intestine, is incorrect. Recent observations have shown that animal 

 matter is preferred to vegetable; all food must be in a state of macera- 

 tion, especial fondness for dead animals being shown. 



Respiration is at first carried on by means of external gills. They 

 are soon replaced by internal structures covered by opercula. 



Rapidity of development depends upon the abundance of food and 

 the temperature of water. Tlie most favorable conditions are a shallow 

 IDOol, readily warmed by the sun and well stocked with organic matter, 

 that is, an old pond. In this stage the frogs may reach a length of 

 several inches, the bullfrog tadpole being largest. The various species 

 closely resemble each other, but can be distinguished after some expe- 

 rience by certain points of mouth structure, size, and coloration. 



In a period varying from two months to two years the first indication 

 of the adult form appears in the protrusion of the two hind legs. The 

 forelegs or arms, owing to their being concealed by the gill membranes, 

 are much later in coming out. 



As the legs become functional the tail is absorbed and furnishes 

 material for growth, so that little food is taken. In the case of the 

 second-year tadpole the capture of insects is begun before the tail is 



