CLEAVAGE 73 



eludes the poles of the egg) will become flattened for two difierent reasons. 

 Firstly, Mitchison claims that there seems always to be some tension in the 

 egg cortex, so that when it expands in one place, the other regions will 

 tend to contract; and secondly, the expansion of the cortex on the egg 

 equator is supposed actually to push the rest of it towards the plane of 

 the spindle equator^. These two processes will, in fact, cause the future 

 furrow not merely to flatten but to bend inwards. This will bring it in 

 contact with the expanding dumbbell-shaped area containing the diffusing 

 active substance, and when this happens the cortex in the furrow will be 

 acted on by the substance and caused to expand; and this expansion com- 

 pletes the formation of the cleavage plane. 



This theory was worked out in the first place from studies on echino- 

 derm eggs. In that form there is a fair amount of support for various 

 parts of it. Thus Mitchison and Swann have presented evidence that the 

 cortical changes leading to cleavage begin in the equatorial regions of the 

 egg, opposite the poles of the spindle, in the form of a decrease of the 

 birefringence and hght-scattering properties of the cortical material. 

 Although the structure of the cortex is not at all well understood, it is 

 reasonable to suppose, as IVlitchison suggests, that it is made up largely of 

 protein chains folded at right-angles to the surface, and that the changes in 

 birefringence are connected with an expansion in area. Again, Dan 

 and Ono (1954, see Dan 1948, 1954) have followed the movements 

 of small particles of kaolin attached to the cell surface. They found 

 that there was an expansion starting opposite the poles of the spindle and 

 spreading towards the future furrow. The furrow region itself at first 

 contracts somewhat, and then expands greatly as the new plane of division 

 cuts down into the depth of the cell. 



Finally, Mitchison and Swann (1955) have devised an apparatus for 

 measuring the deformabihty of the cell surface. The tip of a small pipette 

 is brought against it and a negative pressure or suction appHed; one can 

 then measure the height of the small protuberance which the suction 

 raises on the surface. With this apparatus they showed that shortly before 

 division the cortex becomes much less easy to deform. They suggest 

 that this is not due to an increase in a tension (such as a surface tension) in 

 the cortex, but rather to that layer becoming stiffer or less plastic. They 

 argue that this makes it easier to suppose that the expanding regions 

 opposite the poles of the spindle can successfully push the rest of the cortex 

 into the furrow region. 



^ The student is warned to beware of the verbal pitfalls that can occur from the fact 

 that the long axis of the first cleavage spindle is perpendicular to the animal-vegetative 

 axis of the egg. 



