176 



BIG E LOW. 



[Vol. II. 



A study of sections of the ova at various stages in the first 

 cleavage shows that the mitotic spindle is at first oblique to the 

 long axis of the ovum and nearly perpendicular to the plane 

 in which the furrow first appears (Fig. 5). As cleavage pro- 

 gresses, the spindle turns with the plane of cleavage and at 

 last comes to lie in the long axis of the partially divided ovum 

 (Figs. 6 and 7). The amount of its rotation is approximately 



Fig. 5. 



Fig. 6. 



Fig. 7. 



Drawings of sections of ova in stages of cleavage corresponding approximately with 

 those sliown in Figs, i, 2, and 4, respectively. 



equal to that of the cleavage plane. From what follows it will 

 appear that the rotation of the spindle is produced by the 

 movement of the dividing cell-body. 



The rotation of the first cleavage plane appears to be second- 

 ary to the cleavage processes and capable of an explanation 

 along mechanical lines. The cleavage furrow appears in an 

 almost longitudinal position, passing through the animal pole. 

 As the furrow deepens, the forming cells tend to become 

 spherical and hence lengthen the axis of the ovum perpendicular 

 to the plane of cleavage. If no firm envelope confined the 

 ovum and interfered with change in its form, the long axis of 

 the two-cell stage would lie perpendicular to the plane in which 

 cleavage begins ; but the vitelline membrane evidently interferes 

 with extension in this direction. As the cleavage progresses, 

 therefore, and the resulting cells become more and niore spheri- 

 cal (Figs. 2-3), a rotation of the ovum becomes necessary, for 

 evidently the long axis of the two-cell stage must approximately 

 coincide with that axis of the vitelline membrane. An examina- 

 tion of the figures makes it appear that, as the forming blas- 

 tomeres become more spherical and consequently lengthen the 



