THE BIOLOGY OF THE CELL SURFACE 



As regards the first named finding, that there are two 

 periods of susceptibility in a cleavage-cycle, I should 

 like to point out that an interpretation based on the 

 occurrence of the second which overlooks the first is diffi- 

 cult to defend. If susceptibility as such is the cause for 

 cleavage, then the egg should cleave after each such period. 

 This is not the case. Further, it must be remembered that 

 in many eggs the most pronounced period of susceptibility 

 occurs within a minute after fertilization — that is, depend- 

 ing upon the species of egg, anywhere from i^g or M20 of 

 the total time to first cleavage, at a time long before the 

 onset of the first cleavage-cycle which, as stated above, 

 begins only with the stage of apposition of the egg- and 

 sperm-nuclei. Also if we attempt to correlate the sus- 

 ceptibility with nuclear phenomena, we find ourselves at a 

 loss. During this period of susceptibility immediately fol- 

 lowing fertilization, neither the sperm-nucleus nor the egg- 

 nucleus Is in active mitotic condition. In that period of 

 susceptibility which occurs first in a cleavage-cycle, the 

 apposed nuclei are not in mitosis. Thus only in that sus- 

 ceptible period immediately prior to cleavage is there 

 present an active mitosis. Then susceptibility does not 

 depend upon or run with a definite stage in mitosis. It 

 follows from this that all the learned disquisitions relating 

 the origin of the period of susceptibility to the nuclear 

 state and more specifically to the behavior of the asters 

 have no basis in fact. 



Whilst it is true that the periods of susceptibility can not 

 be correlated with mitotic phenomena, they can be with 

 ectoplasmic state or behavior. At the first period of sus- 

 ceptibilit}' that appears during a cleavage-c}xle, the ecto- 

 plasm manifests peculiar behavior as it does during the 

 period of susceptibility immediateh" prior to the act of 

 clea^'age. The greatest observed susceptibilit}' is that 

 occurring immediatel)' after fertilization; in this stage the 



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