CH. VIII] PFLUGER'S EXPERIMEXTS 85 



though no change had taken place, i.e. nearer the former upper 

 pole. But if the egg had been rotated one hour after fertiliza- 

 tion (or even after the first cleavage), the third furrows would ap- 

 pear on the new upper hemisphere, i.e. nearer the present upper 

 pole. In this last case the four upper cells resulting from the 

 third division are smaller than the lower four. This shows that 

 the four upper cells of the normal eight-cell stage are smaller, 

 not because they are black, but, according to Pfliiger, on account 

 of their position in relation to the force of gravity. Embryos de- 

 velop from these eggs, but they show many abnormal structures. 

 Pfliiger also rotated eggs through 180 degrees after the third 

 cleavage had come in. In four hours and a half the cells of the 

 new upper hemisphere were of the same size as those of the new 

 lower hemisphere. A normal egg at this time would have 

 shown a great difference in the size of the cells of the upper and 

 lower hemispheres. It follows from the last experiments that 

 gravity may affect not only the first, second, and third cleavage- 

 planes, but the later stages as well. " Gravity," Pfliiger said, 

 " according to some unknown law regulates the cleavage-planes. 

 A simple explanation of the phenomenon does not seem possible 

 in the light of the facts." 



THE RELATION OF THE PLAXES OF CLEAVAGE TO THE AXES 

 OF THE EMBRYO 



Pfliiger made other experiments to determine whether, under 

 normal conditions, there exists any relation between the planes 

 of cleavage and the axes of the embryo. He placed seventeen 

 eggs in as many watch-glasses and added water containing 

 sperm. The axes of the eggs were vertical. 



The direction of the plane of first cleavage was noted and 

 marked by a line scratched on each glass. The beginning of 

 the nervous system appeared in about forty-eight hours. In 

 twelve eggs the median plane of the body coincided with the 

 first cleavage-plane, or at most the two planes did not differ 

 more than 10 degrees. In four eggs there was an angle of 

 30 to 60 degrees between the two planes, and in one egg one 

 of 90 degrees. Pfliiger concludes that it is highly probable 

 from this result that the plane of the first cleavage and the 



