24 DEVELOPMENT OF THE FROG'S EGG [Cn. II 



in forcing its way through the inner coat and into the egg, 

 changes then take place in the egg that prevent or make 

 difficult the further entrance of other spermatozoa. The con- 

 traction of the egg, noted above, may possibly have something 

 to do with the process. If, however, two or more sper- 

 matozoa should reach the surface of the egg at about the 

 same moment, it is not improbable that more than one might 

 enter. 1 Both may then pass toward the female pronucleus, but 

 in the frog it is probable that after one male pronucleus has 

 fused with the female pronucleus, the further progress of other 

 male pronuclei that happen to get into the egg is stopped. 



It is sometimes said that the female pronucleus attracts the 

 male pronucleus, but the approach of the two may be due to 

 changes in the protoplasm ; for the migration of the pronuclei 

 through the egg is probably in most cases brought about by 

 the protoplasm of the egg under the influence of the pronuclei, 

 and the pronuclei themselves are merely passively carried 

 along. 



In the newt (Jordan, '93) it seems to be usual for more than 

 one spermatozoon to enter, but only one of these fuses with the 

 female pronucleus. The others subsequently degenerate and 

 go to pieces. In the eggs of other animals, as the starfish, 

 polyspermy, or the entrance of more than one spermatozoon 

 into the egg, brings about disastrous results, causing irregular 

 division of the nucleus and subsequent irregularities in the 

 segmentation of the egg. In these eggs the field of action is 

 small, and the male pronuclei or their centrosomes mutually 

 influence one another and the female pronucleus. In large 

 eggs with much yolk, such as those of the Amphibia and of the 

 Sauropsida, the spermatozoa may be too far apart to affect one 

 another or the segmentation-nucleus, and after the fusion of 

 one male pronucleus with the female the movement of the 

 other male pronuclei towards the female pronucleus seems to 

 stop. 



The head of the spermatozoon enters the egg to become the 

 male pronucleus. The tail of the spermatozoon is left at the 



*It is probable that KupfEer's ('82) account does not apply to eggs under 

 normal conditions. 



