6o 



BIOLOGICAL LECTURES. 



centre of the egg, and always towards the side opposite to that 

 in which the sperm-nucleus lies (Fig. 15). Thus the egg-nucleus 

 first moves away from the sperm-nucleus, which immediately 

 takes up its march in an amoeboid manner towards the centre 

 of the egg, and in the general direction of the egg-nucleus. I 

 may finish at once with the movements of the germ-nuclei by 



saying that they ultimately 

 come together in the cen- 

 tre of the egg. Whether 

 or not the first movement 

 of the egg-nucleus away 

 from the sperm-nucleus is 

 passively brought about by 

 the sphere-substance, the 

 behavior of the germ-nuclei 

 shows that their meeting 

 is not entirely due to mu- 

 tual attraction, but partly, 

 at least, to a common tend- 

 ency to seek a dynamic cen- 

 tre of the egg. 



To. return to the sphere- 

 substance : its invariable migration towards the side of the 

 egg opposite to that in which the sperm-nucleus lies indicates 

 one of two things, either that the sperm-nucleus has driven 

 it away, or else that it is moving along lines of orientation of 

 the egg-substance. To suppose that the minute sperm-nu- 

 cleus could exercise such an effect seems impossible, and the 

 corollary of the second alternative is that the sperm-nucleus 

 has occupied throughout its entire resting period a definite 

 position within the egg, which can only be explained on the 

 assumption of a definite orientation of the egg at the time of 

 fertilization, not only polar, but also corresponding to one of 

 the other chief axes of the embryo. The first cleavage-plane 

 passes very nearly through the point at which the sperm- 

 nucleus has been resting during the maturation. 



During its migration towards one side of the egg, the sphere- 

 substance has undergone a remarkable change of form; it has 



FIG. 15. Unto. Early movements of the germ- 

 nuclei and of the sphere-substance. 



