CLEAVAGE 



89 



result of these inner pulling forces. The facts that the radiating "ten- 

 sion" lines are not always developed, that they vary in different eggs, 

 and that they can be chemically obliterated without interfering with 

 the cleavage indicate that they are not the forces involved in cleavage 

 but that they result from such forces. As the furrow is deepened and 

 becomes fully formed, these lines disappear, and the surface of the egg 



The first cleavage. Tension lines in the cortex during 

 furrowing. 



again becomes smooth. Similar though less pronounced lines gen- 

 erally are seen in association with the second and subsequent cleav- 

 ages. 



The furrow is at first very shallow (superficial), extending from the 

 center of the animal hemisphere around the egg toward the vegetal 

 hemisphere. Its greatest depth is where it is first seen, in the animal 

 hemisphere. Within about 3 hours after fertilization the entire egg is 

 ringed by a vertical furrow. This is always slightly deeper at the ani- 

 mal than at the vegetal hemisphere, and is clearly marked by the 

 concentration of surface pigment. Such grading of the furrow may be 

 due to the mechanical factor of the more resistant yolk at the vegetal 

 pole. In most, but not in all, cases the first cleavage cuts through the 



