II CLEAVAGE 65 



of the two parts of the whole path diverge to a greater or 

 less degree. 



Now it is upon the second part of the path that the posi- 

 tion of the first furrow depends, since it falls in the equator 

 of the fertilization spindle, and the spindle is developed 

 between the two centrosomes which are produced by the 

 division of the sperm centrosome at right angles to that 

 meridional plane which includes the line of union of the 

 pronuclei. Hence, while the first furrow may and does in- 

 deed tend to coincide with the plane of symmetry and the 

 sagittal plane, it need not do so. 



One more point must be briefly alluded to. As has been 

 mentioned the correlation between the sagittal plane and the 

 plane of symmetry is not complete, even when disturbing 

 agencies are removed. At the same time there is some cor- 

 relation between the first furrow and the sagittal plane. It 

 is worth while suggesting that the elongation of the spindle 

 in a certain direction with the concomitant radiation of its 

 asters, may itself impose some bilateral structure upon the 

 egg material, and that the plane of embryonic symmetry may 

 be a resultant of the separate influences exerted by the mitotic 

 figure and the sperm-entrance. 



With regard to the production of the grey crescent, which 

 is due to the retreat of superficial pigment into the interior, 

 it seems probable that this in turn depends on streaming 

 movements in the cytoplasm set up by the aggregation of 

 a watery material to form the entrance funnel. 



We turn next to the theory of the qualitative division of 

 the nucleus. Experiment compels us to reject this also. For 

 if the egg of the Frog be subjected to pressure the sequence 

 of the divisions is altered. When, for instance, the pressure 

 is in the direction of the axis, while the first two furrows 

 preserve their normal meridional directions at right angles 

 to one another, the third furrows, instead of being latitudinal 

 are again meridional or parallel to the first, while the fourth 

 are latitudinal or parallel to the second. It follows that the 

 arrangement of the nuclei produced by these successive 

 divisions is also abnormal, and if the nuclei were really 



F 



