No. 4-] THE FIRST CLEAVAGE OF LEPAS. I 77 



axis of the ovum perpendicular to the plane of cleavage, pressure 

 is obliquely applied to the vitelline membrane with the result 

 that the ovum as a whole rotates, and gradually the dividing 

 ovum adjusts itself to the form of the vitelline membrane. 

 The cleavage plane becomes transverse or oblique, depending 

 upon the amount of rotation necessary to meet adjustment. 

 With a relatively wide vitelline membrane the rotation is less 

 than 90°, for the divided ovum can then become adjusted to 

 an oblique axis of the membrane, and the cleavage plane con- 

 sequently remains oblique. 



The observations recorded above have been repeated on 

 many ova from different individuals, so there can be no doubt 

 that a normal condition is described. 



Marine Biological Laboratory, 



Woods Holl, Mass., 



August 30, 1898. 



