2 Vertebrate Embryology 



in a certain state of development, this may be 

 accomplished by keeping the aquarium in a cool 

 place or by putting lumps of ice in the water. 



During the act of spawning, which may last 

 several days, the male clasps the female firmly 

 with his fore legs and fertilizes the eggs as 

 they leave the cloaca. With the act of spawn- 

 ing the parental instinct ceases, and the eggs 

 are left to themselves, to develop or perish as 

 the case may be. 



As in the laboratory, so in nature, the rate 

 of development depends upon the temperature 

 of the water, but in a few days or a week the 

 eggs have lost their spherical form and have be- 

 come ovoid in shape; and in about ten days the 

 head, body, and tail are marked off from each 

 other by slight constrictions. The embryo now 

 elongates rapidly and by the end of the second 

 week is provided with three pairs of tiny ex- 

 ternal gills, and is able to work its way out of 

 the jelly-like mass with which it is surrounded, 

 and to swim freely in the water. At this time 

 it has no true mouth, and so is dependent, for 

 growth, upon the granules of yolk which were 

 contained in the egg. For several days, until 

 the appearance of the mouth, the tadpole is pro- 

 vided with a horseshoe-shaped sucker on the 



