CH. VIII] PFLUGER'S EXPERIMENTS 83 



axis. It follows always the direction of the force of gravity, 

 i.e. it passes through the vertical diameter of the egg. 



The second cleavage also is vertical, and its position is also 

 determined by the position of the egg, and by the position of 

 the plane of the first cleavage. The third cleavage-planes often 

 show irregularities. Generally they are at right angles to the 

 first two, and lie nearer the upper pole of the egg, or, in other 

 words, their position is also influenced by the force of gravity, 

 for they lie nearer to the pole that stands uppermost at the 

 time. It is a remarkable fact that the subsequent cleavages 'are 

 more rapid in the upper than in the lower hemisphere, no matter 

 what region of the egg has been placed uppermost. Embryos 

 develop from these eggs that have been turned into abnormal 

 positions, and the embryos differ from normal embryos only in 

 the relative distribution of pigment over the surface of the 

 body. Many have the upper surface of the body a light brown 

 color with dark spots ; others have the head, the back, and 

 upper surface of the tail almost free from pigment, and of a 

 whitish-yellow color. The belly in these embryos is more or 

 less deeply pigmented. In a few days, however, new pigment 

 develops over the dorsal surface of the embryo. It should be 

 noted that these paler embryos often show abnormalities, such 

 as bizarre excrescences, irregular movements, slower develop- 

 ment, and that after a few days they begin to die. 



Pfliiger concluded from his experiments that an egg may be 

 divided in all possible directions by the early cleavage-planes 

 according to the position in which the experimenter places the 

 egg, and from such an egg a normal tadpole may develop. 



It is not, however, entirely a matter of indifference what angle 

 is made between the cleavage-planes and the primary axes. It 

 is certain that if the upturned hemisphere contains more white 

 than black, a normal embryo may develop ; but if the upturned 

 hemisphere be entirely white, i.e. if the egg has been rotated 

 through 180 degrees, embryos may occasionally develop, but 

 they are nearly always abnormal and soon die. It is difficult, in 

 fact almost impossible, to keep the white hemisphere upward ; 

 for in nearly every case Pfliiger found that later a partial rota- 

 tion of the egg took place, so that a crescent of black appeared 

 above the horizon. One exceptional case is worth recording. 



