CHAPTER IV 



CLEAVAGE OF THE EGG 



When the egg comes to rest in its membranes after fertiliza- 

 tion has taken place, it will be found that the egg-axis assumes 

 an oblique position with respect to the vertical. The degree of 

 obliquity may be different for the eggs of different species of 

 frogs, but in some species it is carried so far that, when the 

 egg is looked at from above, a crescent of the white hemisphere 

 can be seen on one side of the egg. Roux has stated that the 

 declination of the egg-axis takes place only after the entrance 

 of the spermatozoon, and toward that side into which the sper- 

 matozoon has penetrated. 1 He was able to determine this by 

 artificially fertilizing the egg at definite points. By means of 

 a small pipette, water containing spermatozoa was brought in 

 contact with the jelly somewhere near the upper hemisphere of 

 an egg. Presumably the spermatozoon will then take the short- 

 est path to the egg. Roux found that the egg after a time gen- 

 erally rotated on its axis toward the point at which the artificial 

 fertilization was supposed to have taken place. 



Normal Cleavage 



The first furrow appears on the egg about two and one half 

 to three hours after fertilization, the time depending in part on 

 the temperature of the water. A rather wide furrow appears 

 in the flattened area near the black pole, and rapidly extends 

 over the upper surface of the egg, and then moves more 

 slowly over the lower or white surface. The sides of the 

 furrow are often wrinkled, probably a mechanical result of the 



1 Roux believes the obliquity to be a usual phenomenon after fertilization 

 for some species ; in others the obliquity is only occasionally seen. Schultze 

 finds it to be as much as forty-five degrees in Rana fusca. 



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