THE BIOLOGY OF THE CELL SURFACE 



chromosomes are at the anaphase and the egg is still spheri- 

 cal, the hyaline plasma-layer is everywhere of equal width 

 and the filaments in it are all the same length. In later 

 anaphase or early telophase the ectoplasm shows a rapid 

 change, moving from over the spindle poles toward the 

 equator, the site of the future cleavage-plane, in an amoe- 

 boid or wave-like fashion. The ectoplasm then begins to 

 move inward toward the egg-centre, in the direction of the 

 course of the cytoplasmic currents, and the egg elongates. 

 As the ectoplasm continues to move inward, the covering 

 film of the hyaline plasma-layer resists. Thus the moving 

 ectoplasm exerts a pull on its filaments in the region of the 

 oncoming cleavage plane so that they are longer here than 

 elsewhere. With the beginning of the moving of the wave 

 of amoeboid changes from the area over the spindle poles 

 to the equator, the filaments in the former region appear 

 to lose connections with their covering membrane, whilst 

 those in the latter seem more firmly bound to it. 



By the very simple method of treatment with extremely 

 dilute sea-water, I was able to ascertain that while the 

 egg is still spherical and when the ectoplasm exhibits 

 the wave-like movement from the circumpolar areas to the 

 equator, it has definitely localized weaknesses in this, the 

 most susceptible, period of its cleavage-cycle. At this time 

 the egg breaks down in about ten seconds after immersion 

 in the dilute sea-water by an outflow from the areas over 

 the spindle-poles. Though eggs of some species of sea- 

 urchins show this localized disruption more clearly than 

 others, it can be observed in all; none ever show an exuda- 

 tion elsewhere than from the circumpolar areas. With 

 elongation the egg is again resistant. 



As we have seen, the eggs' period of greatest susceptibility 

 to hypotonic sea-water occurs during the process of mem- 

 brane-separation. At this time the ectoplasmic filaments 

 appear more sharply because of break-down of material 



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