POLARITY AND BILATERALITY OF THE ANNELID EGG. 57 



trifuging. Now, assuming that there is no polarity of the ground 

 substance, as this hypothesis requires, the spindle would naturally 

 follow the shortest path to the surface, for this would usually be 

 the path of least resistance. In other words, the position given 

 the nucleus or the spindle by the centrifugal force would deter- 

 mine the polarity in all cases, because its position is always 

 excentric. However, this is not the case, as the following facts 

 demonstrate : 



i. Most of the experiments on the egg of Chcetopterus were 

 made before fertilization at the stage of the mesophase of the first 

 maturation spindle. All the ripe eggs of this animal attain this 

 stage without fertilization, and the spindle comes to rest and 

 remains at the mesophase unless the egg be fertilized or other- 

 wise effectively stimulated (Fig. 4). The spindle is attached to 

 the surface and its axis is in line with the axis of the egg. If the 

 direction of the centrifugal force is such that the spindle lies 

 in the central hemisphere, i. e., in the half of the egg towards 

 the post, it may remain attached, but is often detached from the 

 surface, and in such cases it always lies in the clear band. If the 

 spindle pole of the egg lies in the distal hemisphere during cen- 

 trifuging, it is always detached from the surface and comes to rest 

 in the clear band. The result therefore is always the same so 

 far as the position of the spindle is concerned : the spindle lies 

 invariably in the clear band after centrifuging, either attached to 

 the surface, or free. 



Nevertheless, the polar bodies may form at any point in the 

 yolk hemisphere, as is demonstrated by study of the living eggs, 

 and also by sections of series of eggs preserved during the 

 maturation period after centrifuging. Figures of such living 

 eggs are given in the paper already referred to, and a section of 

 such an egg is reproduced here (Fig. 2). When one considers the 

 resistance that has to be encountered by the spindle in making its 

 way through the densely packed yolk-granules of the distal 

 hemisphere, and remembers that a shorter path to the surface 

 through hyaline protoplasm was open, it is obvious not only that 

 the spindle does not follow the shortest path to the surface, but 

 also that a factor of great intensity has been in operation. 



I stated in my previous paper ('06) that the formation of polar 



