OF VITAL PHENOMENA 157 



That the surface tension at the equator is greater than at the 

 poles of the dividing cell is demonstrated by the superficial move- 

 ment of the protoplasm. When the tension of a surface is locally 

 altered surface currents pass from the regions of low tension 

 to regions of high tension. Gardiner (1897) observed the super- 

 ficial granules in certain dividing eggs to pass in the direction 

 from the poles to the equator. The superficial granules in the 

 Arbacia egg become heaped up at the equator during cell division 

 (McClenidon, 1910 e), and must have moved in this direction, 

 since they are evenly distributed before division commences. 

 Conklin (1908) observed somewhat similar movements in the 

 eggs of Crepidula. 



It is difficult to make a model of cell division because it is 

 difficult to maintain local differences in surface tension long 

 enough for complete division to take place. The following model 

 requires considerable skill to manipulate. A drop composed of 

 one part chloroform and two parts rancid olive oil is immersed in 

 water and prevented from touching the bottom by a layer of 

 concentrated salt solution under it. Two pipettes filled with .1 

 normal alkali are placed near opposite poles of the drop and the 

 alkali allowed to flow onto these two poles simultaneously. The 

 drop bulges at the poles and constricts in the middle. The re- 

 duction of the surface tension at the poles causes surface cur- 

 rents toward the equator, which spread the alkali over the whole 

 surface of the drop. In some instances, however, division is 

 completed before this is accomplished. 



Fertilisation 



Many egg cells do not divide unless they are fertilized by 

 sperm, and it was formerly believed that they are incapable of 

 division unless fertilized. 



The process of fertilization may be divided into the following 

 factors and stages: first, the finding of the egg by the sperm; 

 second, the change in the egg responsible for cell division; third, 

 the entrance of the spermatozoon into the egg and fusion with 

 the female pronucleus; and fourth, the transmission of the pater- 

 nal characters to the egg. 



F. Lillie (1913) observed that the sperm of Nereis and Arbacia 

 are attracted by substances given out by the eggs. Since he 



