The Development of the Frog 1 7 



of fertilization ; the fate of the middle-piece, 

 in the frog, is not well understood, but it is 

 possible that it may effect segmentation in 

 some way. As a rule, only one spermatozoon 

 enters the egg, but it is likely that if two or 

 more spermatozoa reach the vitelline mem- 

 brane at the same time, they may all enter the 

 egg, although only one male pronucleus will 

 fuse with the female pronucleus. In some 

 other animals the entrance of two or more 

 spermatozoa into the egg (polyspermy) pro- 

 duces serious results, causing irregularities in 

 segmentation ; but in the frog the extra pro- 

 nuclei probably disappear without producing 

 any unusual effect. 



Segmentation of the Egg 



About two or three hours (depending on 

 the temperature) after fertilization, the first in- 

 dication of segmentation is seen as a furrow on 

 the dark pole of the egg. This furrow gradu- 

 ally extends around towards the white pole 

 until it completely encircles the egg (Fig. i, 

 A). By the time this has taken place, the 

 contents of the egg have been separated into 

 two parts by a plane corresponding to the 

 superficial furrow, so that the egg is now 



