ADAPTATION IN CLEAVAGE. 



61 



FIG. 16. Unto. Outline of section in the direction 

 of the line ruled across Fig. 15 ; to show the elon- 

 gation of the sphere-substance. 



elongated greatly in a Jiorizontal direction, at rigJit angles to 

 the line uniting the germ-nuclei (Fig. i6)> and has tJius marked 

 out the direction of the first 

 cleavage-spindle. Now this 

 elongation has begun, and 

 the plane of division is indi- 

 cated before the germ-nuclei 

 have met. 



It might very readily be 

 assumed that the plane of 

 the first cleavage is deter- 

 mined by the copulation- 

 path of the germ-nuclei, as 

 is stated to be the case in 

 the ova of some other ani- 

 mals (frog, Roux ; Toxo- 

 pneustes, E. B. Wilson). 

 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 first 



cleavage-spindle before the 

 germ-nuclei come together. 

 It would be necessary, I 

 believe, to assume that the 

 distant sperm-nucleus ex- 

 ercises an influence on the 

 direction of migration and 

 of elongation of the sphere- 

 substance in the first cleav- 

 age, but that, in the next 

 division, the sphere-sub- 

 stance acts independently; 

 and this assumption is ab- 

 surd on the face of it. 



The first cleavage-spindle 

 forms in the centre of the 

 egg (Fig. 17), in the plane already indicated by the elongation 

 of the sphere-substance. At first there is a single very minute 



FIG. 17. Unio. First cleavage-spindle forming. Basi- 

 chromatin granules near the ends of the spindle ; oxy- 

 chromatin granules on the spindle-fibres. 



