Ch. IX] EXPERiaiENTS OF BORX AND OF ROUX 91 



live minutes after inversion a rearrangement of tlie contents 

 has begun. The heavier white yoUc has begun to sink down 

 on one side, taking the shortest path toward the bottom of 

 the inverted egg. As the heavier yolk sinks down in response 

 to the action of the force of gravity, the granuhir protoplasm 

 rises up on the opposite side. The two sorts of substances do 

 not mix during the interchange of position, but keep sharply 

 separated from each other. The pigment-rind remains fixed, 

 but loses something of its thickness. After an interval of 

 forty-five minutes to two hours, the finely granular protoplasm 

 has reached the highest point of the egg, and has spread out 

 under the surface of the white hemisphere. The yolk has 

 passed to the lower hemisphere of the inverted egg, and now 

 lies inside of the black rind. 



This description of the movement of the contents of the egg 

 applies to all those cases in which the white pole does not stand 

 exactly upward, or, in other words, where the egg is turned 

 less than 180 degrees. When the egg is completely inverted 

 the force of gravity causes the contents of the egg to rearrange 

 themselves in a somewhat different way. The yolk sinks down 

 on all sides, while the lighter protoplasm rises up through the 

 centre of the egg, carrying with it the nucleus. 



Pfliiger believed that eggs which had been rotated through 

 180 degrees, and kept in that position, did not segment because 

 of the covering up of a micropyle, where the black pole came 

 in contact with the lower surface of support. Born thinks 

 that such eggs do not segment, owing to the inability of the 

 spermatozoa to pierce the white rind which is uppermost.^ 



In the normal egg the path of the spermatozoon can be fol- 

 lowed by the trail of pigment passing in from the surface of 

 the egg, which marks the direction taken by the spermatozoon. 

 This pigment-line can also be followed in the partially inverted 

 egg, and it is seen that the male pronucleus is also carried 

 along in the streaming protoplasm. 



Born found in eggs that have been partially inverted, that 

 the first cleavage-plane is generally vertical, passing through 



^ The problem of the extrusion of the second polar body in these eggs should 

 be examined. 



