INTRODUCTION 



The eggs of most of our species of frogs are laid in the 

 spring. In some cases they are set free ahuost immediately on 

 the emergence of the frogs from their winter sleep ; in other 

 cases the eggs are not laid until some weeks or even months 

 after the frogs have awakened. In almost every instance the 

 eggs are deposited in water and usually in quiet pools or ponds, 

 or in protected bays along streams where the water has backed 

 up and has come to rest. Sometimes the bunches of eggs are 

 stuck to sticks, grass, submerged sedge, or even to stones ; in 

 other cases the bunches are not fastened. 



The copulation precedes and lasts through the laying-period ; 

 a single male fertilizing all the eggs laid by one female. The 

 sperm pours out of the cloaca of the male at the moment when 

 the eggs pass out of the female. 



Both the male and the female sexual products, the eggs and 

 spermatozoa, are ripened during the summer and autumn of 

 the year preceding the deposition of the eggs, — at least this 

 is the more usual process. The origin of these sexual cells 

 must first be studied in order to more fully understand their 

 relation to each other, and the part they play in the subsequent 

 development. 



XI 



