28 DEVELOPMENT OF THE FROG'S EGG [Cii. Ill 



of its own and of other species. If the head is large, the sper- 

 matozoon can force its way only into those eggs that are adapted 

 to spermatozoa with large heads. For instance, the sperma- 

 tozoa of R. fusca have thinner heads than any others, and 

 the head is, moreover, very pointed. These spermatozoa can 

 fertilize eggs of nearly all other species (R. arvalis, R. escu- 

 lenta, B. communis). Conversely, the thick-headed spermatozoa 

 of R. arvalis and the blunt-headed spermatozoa of R. esculenta 

 cannot get into the eggs of R. fusca. 



The spermatozoon of B. communis, which has a very pointed 

 but somewhat larger head than that of R. fusca, appears never- 

 theless to be able at times to penetrate the eggs of R. fusca and 

 to fertilize them. That the spermatozoon of Triton can enter 

 the eggs of R. fusca is explained very easily when we remember 

 that the sharp thin head of the Triton spermatozoon is best 

 adapted of all species to penetrate any egg. We see, too, that 

 the thick-headed spermatozoon with a blunt anterior end, such 

 as those of R. arvalis and R. esculenta, cannot fertilize the eggs 

 of any other species. And finally, to confirm the conclusion, 

 we find that these two species, R. arvalis and R. esculenta, 

 which have large-headed spermatozoa, are alone capable of 

 reciprocal crossing. .Pfliiger believed that the eggs have the 

 greatest capacity for cross-fertilization at the height of the 

 breeding season, and the same statement holds, but in a much 

 less degree, for the spermatozoa. 



Experiments on Other Forms 



Hertwig has objected to Pfluger's conclusions on the ground 

 that the eggs of the sea-urchin are much more capable of cross- 

 fertilization after they have begun to suffer change either from 

 being kept some time in sea- water, or from the application of 

 drugs. He thought that the frogs kept by Pfliiger had been 

 also under artificial conditions. Further, Hertwig concluded, 

 from his results on sea-urchins, that the possibility of crossing 

 does not depend entirely upon the external conditions, but to 

 a large extent upon some unknown property of the egg. Eggs 

 in good condition are able to prevent the entrance of foreign 

 spermatozoa, but as soon as they begin to lose their irritability, 

 they can no longer resist the entrance. 



