II HABITS AND NATURAL HISTORY OF THE FROG 49 



killed near the end of their breeding period have nearly all 

 the eggs in the uterus. At the beginning of the breeding 

 period the seminal vesicles of the males are empty, or con- 

 tain very little seminal fluid. Steinach found on examining 

 a large number of copulating specimens of Rana temporaria^ 

 which he received March 5, 1893, that the seminal vesicles 

 contained no traces of spermatozoa. Only on March 8 were 

 the seminal vesicles in some specimens partly filled, and 

 there were yet several males in which the vesicles were 

 empty on March 9. Copulation begins, therefore, several 

 days before the sexual products of either sex are ready for 

 discharge. 



When the eggs are extruded through the cloaca of the 

 female, the male discharges his spermatic fluid over them, 

 and the spermatozoa penetrate the jelly around the eggs and 

 complete the act of fertilization. The operation is some- 

 what similar to the fertihzation of the eggs of most fishes, 

 where the male who accompanies the female during the breed- 

 ing period discharges his milt or sperm over the eggs at the 

 moment when they are extruded. The clasping instinct of 

 the male frog serves to insure the proximity of the two sexes 

 when the proper moment for fertilizing the eggs arrives. 

 When the time for extruding the sexual product arrives, the 

 pair sink to the bottom of the water, where they remain 

 quiet until the sexual products are discharged, when they 

 separate. The male then loses his clasping instinct and is 

 totally indifferent to the other sex. As a rule he fertilizes 

 but one batch of eggs a year. 



Fischer-Sigwart is of the opinion that all of the eggs are 

 not generally fertilized at the time of their extrusion, but 

 that many are fertilized later by other males, which are usu- 

 ally found among the masses of eggs in the breeding places. 

 This observer has often seen males discharging their fluid 



E 



