CHAPTER X 



MODIFICATION OF CLEAVAGE BY COMPRESSIOX OF THE 



EGG 



In 1884 Pfliiger made the important and novel experiment 

 of compressing the unsegmented egg of the frog between paral- 

 lel plates of glass. In consequence, the cleavage was modified, 

 and there was found to be a direct relation established between 

 the planes of cleavage and the direction of tlie pressure applied. 

 The first three planes of division were at right angles to the 

 compressing plate. Pfliiger explained these results as due to 

 the position which the nuclear spindle would take during its 

 elongation. The long axis of the spindle, he thought, would 

 place itself in the direction of least resistance, i.e. in a plane 

 parallel to the glass plates ; and since the division of the cell is 

 at right angles to the long axis of the spindle, it will, therefore, 

 be at right angles to the compressing plates. Horn ("93) and 

 Hertwig ('93) simultaneously repeated Pflliger's experiment, 

 making also some modifications of the original experiment. 

 Horn's account is here followed, as it gives a more detailed 

 report of the results. 



Effect of Compressing the Segmenting Egg between 



Parallel Plates 



The eggs of Rana fusca are on an average 1.5 mm. in diame- 

 ter. The distance between the two glass plates in the experiment 

 was 1.4 mm., for if less the eggs were burst by the pressure. 

 Since all of the jelly around the egg was not removed, the actual 

 diameter of the egg., as subsequent measurements showed, was 

 less than the distance between the two parallel plates (1.4 mm.). 

 For instance, a compressed egg (after it had been killed and 



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