84 DEVELOPMENT OF THE FROG'S EGG [Cii. VIII 



An egg was observed that had its white hemisphere turned 

 exactly upwards until the first cleavage came in. More water 

 was then added, and the egg retained its reversed position and 

 continued to segment energetically and with wonderful regu- 

 larity. The upturned white hemisphere was soon divided into 

 many small cells, while the cells in the lower black hemisphere 

 were larger. A later examination of the egg showed that dur- 

 ing the cleavage the egg had rotated through about 45 degrees, 

 bringing a portion of the black hemisphere above the horizon. 

 Still later the egg seemed to rotate back again into its first re- 

 versed position. After a time the development stopped and the 

 egg died. 



If, as the preceding experiments seem to show, there exists 

 a relation between the force of gravity and the position of the 

 first three cleavage-planes, it is important to know whether 

 gravity acts only at the moment of cleavage or whether the 

 action is a slow and continuous one. Pfluger found that if 

 the egg at the two-cell stage be rotated a few seconds before 

 the appearance of the second furrow, so that a new angle is made 

 by the primary axis with the direction of the force of gravity, 

 then the second furrow comes in as though the egg had not 

 been changed, and may therefore make any possible angle with 

 the direction of the force of gravity. The same experiment 

 can be made if more water is added to an egg that has already 

 segmented once in an abnormal position. The egg may then 

 rotate so that the first cleavage-plane is no longer vertical ; 

 nevertheless, the second furrow always comes in at right angles 

 to the plane of the first furrow, and, therefore, may make any 

 possible angle with the direction of the force of gravity. 



A different result folloAvs if the egg has been rotated one 

 hour after fertilization and therefore some time before the time 

 of the first cleavage. The plane of cleavage of the second divi- 

 sion is then affected, and coincides with the direction of the 

 force of gravity. We must conclude that an interval of one 

 hour at least is necessary to produce any change in the egg. 



What has just been said with regard to the second planes of 

 cleavage holds equally well for the third cleavage-planes. If 

 the egg be rotated through 180 degrees after it has divided 

 twice (into four parts), then the third furrows come in as 



