92 DEVELOPMENT OF THE FROG'S EGG [Cn. IX 



the highest point of the egg. Sometimes, however, the plane of 

 cleavage is oblique to the vertical, i.e. occasionally it does not 

 pass through the highest point of the egg. The position of 

 this vertical or nearly vertical plane of cleavage bears generally 

 some relation to the path, or meridian, of streaming of the 

 contents of the egg. The first plane of cleavage corresponded 

 with the streaming meridian in about one-third of one hundred 

 recorded cases. In nearly all of the remaining two-thirds, the 

 first cleavage-plane stood nearly at right angles to the stream- 

 ing meridian. 1 



Born's results throw a new and important light on Pfliiger's 

 experiments. The force of gravity acts on the rotated egg 

 only to bring about a rearrangement of the contents of the egg 

 in accordance with the specific gravity of the substances pres- 

 ent. This is the only connection between the direction of the 

 force of gravity and the direction of the planes of cleavage. 

 We also see why a certain amount of time is necessary after 

 the reversal of the egg for the rearrangement to take place. 



THE CLEAVAGE OF THE EGG IN A CENTRIFUGAL MACHINE 



Roux ('84) tested in another way the effect of gravity on 

 the segmentation of the frog's egg. "What would happen," 

 he asked, " if an egg were so placed that at every moment a 

 new point was turned uppermost ? Further, if gravity acts 

 only so as to rearrange the contents of the egg, what would 

 take place if a centrifugal force were applied to the eggs before 

 cleavage ? " Such a centrifugal force ought to cause the egg 

 to orient itself in respect to the direction of that force, in the 

 same way that gravity causes the egg to turn. 



A wheel rotating around a horizontal axis was used. To 

 this wheel were attached tin boxes into which the eggs were 

 put. A box could be placed at any point along a radius of the 

 wheel. When the machine made eighty-four revolutions a 

 minute, some of the boxes were so placed that the centrifugal 



1 In this case the second cleavage-plane would correspond with the meridian 

 of streaming. Born states that the median plane of the embryos, developing 

 from the rotated eggs, passes through the secondary meridian that cuts the 

 highest edge of the white field in its partially inverted position. 



