No. 2.] ORGANIZATION OF THE EGG OF UNIO. 247 



a time when the two germ-nuclei are still separated by a quarter 

 of the diameter of the egg. The line of elongation of the sphere 

 substance marks the long axis of the first cleavage spindle. Thus 

 the first cleavage must pass very nearly through the point at wJncJi 

 the sperm-7iucleus has been resting. 



The invariable migration of the sphere substance towards 

 the side of the Q%g opposite to that in which the sperm-nucleus 

 lies indicates one of two things : either that the sperm-nucleus 

 has, so to speak, driven it away, or else that it is moving along 

 lines of orientation of the egg-substance. The first alternative 

 appears to me manifestly absurd, and the corollary of the sec- 

 ond is that the sperm-nucleus has occupied throughout its entire 

 resting period a definite position in the egg ; and this can be 

 explained only on the assumption of a definite orientation of 

 the egg-substance, not only polar but also corresponding to 

 the chief axes of the embryo. 



It might very readily be assumed, on the other hand, that the 

 plane of the first cleavage is determined by the copulation path 

 of the germ-nuclei, as is stated to be the case in the ova of some 

 other animals, frog (Roux, 87) and Toxopneustcs (Wilson, '95). 

 But whoever should take this position for Unio would have to 

 explain how it happens that the sphere substance elongates in 

 the plane of the future first cleavage spindle before the germ- 

 nuclei come together. It would be necessary, I believe, to 

 assume that the distant sperm-nucleus exercises an influence on 

 the direction of elongation of the sphere substance in the first 

 cleavage, although in the next division the sphere substance 

 acts independently. And this assumption is absurd on the 

 face of it. (For discussion of this, see p. 262.) 



In the movements of the germ-nuclei there are two factors 

 concerned : first, certain dynamic relations between the egg-sub- 

 stance and germ-nuclei, under which we include the influence 

 exerted by the orientation of the egg-protoplasm ; and, second, 

 the mutual attraction of the germ-nuclei. The latter controls 

 the course of the copulation path of the germ-nuclei to a great 

 extent, but not altogether. It is generally, hov^^ever, so strong 

 a force that the influence of the former factor is not very 

 apparent during the copulation path, but, after the germ-nuclei 



