THE VITAL PHENOMENA OF THE CELL 



217 



FIG. 109. Egg from Fab- 

 ricia (after Haeckel) : A 

 animal portion; V vegeta- 

 tive portion. 



However, this is only an apparent exception to the law already 

 mentioned, for reflection shows that the nucleus, in seeking to 

 occupy the centre of its sphere of action, 

 only affords an example which confirms the 

 law. Interactions take place between the 

 nucleus and the protoplasm, not between 

 it and the yolk-substance, for the latter 

 during all the processes of division behaves 

 like an inert mass. Thus the unequal dis- 

 tribution of the protoplasm must, in con- 

 sequence of the above law, affect the position 

 of the nucleus, forcing it to make its way 

 to those places where the protoplasm is 

 chiefly collected, that is to say, away from 

 the centre of gravity. The nearer tin 

 latter approaches the vegetative pole, the nearer the cleavage 

 nucleus approaches the animal pole. 



Actual examination shows the truth of this statement. In the 

 Frog's egg (Fig. 115), the cleavage nucleus is somewhat above 

 the equatorial plane of the sphere in the animal half, whilst in 

 eggs, where the protoplasm is more sharply differentiated as a 

 germinal disc from the yolk (Fig. 108), the cleavage nucleus has 

 risen quite close to the animal pole, and has taken up a position 

 inside the germinal disc itself (Reptiles, Birds, Fishes, etc.). 

 Similarly in the egg of Fabricia (Fig. 109), the cleavage nucleus 

 has been pushed towards that portion of the oval body which is 

 rich in protoplasm. 



Further, the reaction between protoplasm and nucleus, affect- 

 ing the position of the latter, becomes more marked from the 

 moment when the poles develop. Thus the second general law 

 may be stated here, that the two poles of the division figure 

 come to lie in the direction of the greatest mass of protoplasm, 

 somewhat in the same way as the poles of a magnet are in- 

 fluenced as to their position by the iron filings in their neigh- 

 bourhood. 



According to the second law, in a spherical egg, for instance, in 

 which protoplasm and yolk are evenly distributed, the axis of the 

 centrally laid nuclear spindle may coincide with the direction of 

 any radius whatever ; whereas, on the contrary, in an oval proto- 

 plasmic body it can only coincide with the longest diameter. In 

 a circular protoplasmic disc the spindle axis is parallel to the 



