274 PRINCIPLES OF STOCK-BREEDING. 



Agassiz has shown that turtles begin to copulate 

 at the age of seven years, but do not lay until they 

 are eleven years old. They copulate twice each year 

 for four years, before the eggs are fully matured. 

 "Upon opening large numbers of young Chrysemys 

 picta, 1 it was ascertained that, up to their seventh year, 

 the ovary contained only eggs of very small size, not 

 distinguishable into sets ; but that with every succeed- 

 ing year there appears in that organ a larger and larger 

 set of eggs, each set made up of the usual average 

 number of eggs which this species lays, so that speci- 

 mens eleven years old for the first time contain ma- 

 ture eggs, ready to be laid in the spring." 



From observations made by Agassiz, " it appears 

 that the first copulation coincides with a new develop- 

 ment of the eggs, in consequence of which a certain 

 number of them, equal to that which the species lays, 

 acquire a larger size, and go on growing for four suc- 

 cessive years before they are laid, while a new set is 

 started every year, at the period of copulation in the 

 spring, enabling this species to lay annually from five 

 to seven eggs after it has reached its eleventh year." a 



After a careful examination of all the known facts 

 bearing upon this interesting subject, Agassiz became 

 satisfied that " the first copulation only determines the 

 further growth of a certain number of eggs, which re- 

 quire a series of successive fecundations to undergo 

 their final development ; " and that " in turtles a repe- 

 tition of the act, twice every year for four successive 

 years, is necessary to determine the final development 



1 A common fresh-water turtle. 



8 " Embryology of the Turtle," by Agassiz, pp. 490, 491. 



