No. 2.] ORGANIZATION OF THE EGG OF UNIO. 255 



IV. Movements of the First Cleavage Spindle; 

 First and Second Cleavages. 



The first cleavage spindle is invariably formed in the exact 

 center of the Q%g, as already described, and parallel to the 

 plane of elongation of the sphere substance, which stretches 

 completely across the Q%g. The spindle remains in the center 

 of the Qg^ until just before the metaphase, and then, without 

 undergoing any considerable change in form or size, it moves 

 end first towards one side of the Q%g, until one sphere comes 

 almost in contact with the cortical alveoli (PI. XXVII, Figs. 45 

 and 46). This movement is a necessary preparation for the 

 first cleavage, which is very unequal. But it is difficult to 

 understand why the spindle should first form in the center of 

 the ^gg and secondarily change its position. The metaphase 

 and almost the entire anaphase (PI. XXVII, Fig. 47) of the 

 spindle are spent in this position, and then the spindle shifts 

 back a little (PI. XXVII, Figs. 48 and 49) to the definitive posi- 

 tion of the cleavage plane. (See descriptions of these figures.) 

 It is possible that there may be other lesser oscillations of the 

 spindle before the point of equilibrium is found. 



It is perfectly plain from this description that the position 

 of the spindle does not primarily determine the place or direc- 

 tion of the first cleavage plane, but that, on the contrary, the 

 position of the spindle is controlled through the cytoplasm, as 

 a needle in a magnetic field oscillates until equilibrium is 

 attained. Now there are no visible differences in structure in 

 different parts of the egg, which might explain the movements 

 of the spindle, but it seems to me that the movements them- 

 selves are an indication of a definite bilateral orientation ; for 

 I cannot conceive it as being possible that in an isotropic sub- 

 stance movements of such definiteness and prospective value 

 in the cleavage should take place. Thus, although there is no 

 other evidence that the movement of the spindle in determin- 

 ing the inequality of the first cleavage is in one direction 

 rather than the other, I am convinced that it is always in 

 the same direction under the influence of some predelineated 

 organization of the cytoplasm. 



