V THE DEVELOPMENT OF THE FROG 89 



bearer of hereditary qualities ; and especially since the 

 cytoplasm in the two germ cells differs so enormously in 

 amount. 



Very soon after the spermatozoon has entered the egg a 

 mass of fluid collects between the egg and vitelline mem- 

 brane. This is called the peri-vitelline fluid, and is doubtless 

 derived from the egg itself. Owing to the accumulation of 

 this fluid the egg becomes free to rotate, and the dark pole, 

 which has a less specific gravity than the light-colored yolk- 

 laden region, soon comes to lie uppermost. Before fertili- 

 zation the eggs he in all possible planes, but soon after 

 fertilization they all assume the same position, with the black 

 pole above. 



Cleavage. — Usually between two and a half and three 

 hours after fertilization the egg begins to undergo its first 

 division. The process begins as a small depression at the 

 animal pole, which gradually extends in the form of a groove 

 until it finally surrounds the egg. This groove marks the 

 outer boundary of a cleavage plane which extends through 

 the egg, dividing it into approximately equal cells. The 

 cleavage of the mass of the egg is preceded and accompanied 

 by a karyokinetic division of the nucleus, so that each daugh- 

 ter cell contains a nucleus derived from the copulation 

 nucleus of the fertilized egg ; and in all subsequent cleavages 

 of the ovum there is a separation of nuclear as well as 

 cytoplasmic material, and chromatin derived from both 

 parents comes to lie in all the cells of the body of the 

 embryo. 



The egg before cleavage is not entirely a radially sym- 

 metrical structure, but the dark pole incliii^s somewhat to 

 one side. There is consequently only one plane which can 

 divide the egg into two symmetrical halves. As a rule the 

 first cleavage furrow lies in this plane of symmetry ; more 



