ADAPTATION IN CLEAVAGE. 



63 



To what extent this centre of equilibrium is determined by 

 the primary orientation of the cytoplasm, and, how much by the 

 secondary distribution of the sphere-substance, it is impossible 

 to say ; but the orientation of the cytoplasm is, in either event, 

 the primary factor, inasmuch as it determines the distribution 

 of the sphere-substance. 



The first cleavage-furrow now forms rapidly, and, as it forms, 

 the spheres undergo an enlargement, migration, and change 

 of shape analogous to that preceding the first cleavage. We 

 shall follow it only in the larger cell. The sphere-substance 





Fig. 19.— Uiiio. Secondary shifting of tlie first cleavage-spindle. Beginning of 

 growth of the sphere-substance. 



in this cell elongates during the early stages of reconstitution of 

 the nucleus in a horizontal direction parallel to the axis later 

 taken by the second cleavage-spindle in this cell (Fig. 20). At 

 the same time the entire cell elongates in the same direction 

 and becomes slightly constricted in a plane and position corre- 

 sponding precisely with the next cleavage-plane ; this, before 

 the nucleus is reconstituted, or has moved away from the neigh- 

 borhood of the first cleavage-wall. It is as though the cytoplasm 

 were making an attempt at division, which is rendered abortive 

 by the stage of development of the nucleus. Later both cells 

 round off and then become applied together, and the second 

 cleavage-spindle forms and moves into the position of the division 

 already indicated. 



