THE BIOLOGY OF THE CELL SURFACE 



resistance and susceptibility to certain experimental agents. 

 As I pointed out in the chapter, General Properties of 

 the Ectoplasm, unfertilized eggs in hypotonic sea-water 

 disintegrate more slowly than eggs which are exposed to 

 the same degree of hypotonicity when after insemination 

 break-down in the ectoplasm takes place. This period of 

 ectoplasmic changes having passed over, the eggs are more 

 resistant than unfertilized ones. According to many 

 observers this resistance persists until just before or at 

 first cleavage when the eggs become susceptible. It is 

 held by some that this susceptibility is due to increased 

 surface-tension over the spindle-poles. 



Now sea-urchins' eggs, as we have seen, are first spheres, 

 then ellipsoids and finally, by cleavage, two spheres. Does 

 the supposed change in surface-tension take place while the 

 egg is still a sphere, when it becomes an ellipsoid or as it 

 forms two spheres .^ Or does the theory disregard entirely 

 that stage during which the egg elongates in the direction 

 of the spindle-axis .'' The theory demands the most exact 

 fixing of the moment in the cleavage-cycle when the egg 

 reveals its maximum susceptibility in order to relate this 

 to the act of cleavage. Observations made too far apart 

 on a lot of eggs developing at the same tempo could easily 

 miss the moment of maximum susceptibility. If, on the 

 other hand, closely set observations be made on poor eggs 

 developing at varying rates and therefore not reaching 

 first cleavage at the same instant, the moment of maximum 

 susceptibility would be erroneously fixed as coming in the 

 stage or stages of the intact eggs present whilst the disin- 

 tegrated ones would be in other stages.^ 



^ Degree of hypotonicity should be great enough to give sharp 

 results; transfer of eggs should be uniform as to number of eggs, 

 amount of dilute sea-water and amount of normal sea-water into 

 which eggs are transferred. 



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