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whitman: - [Vol. I. 



to the periphery of the egg during the period of maturation? 

 As this centrifugal movement may be upward, downward, or 

 to one side, even in the same class of eggs (yC.g., Teleostei), it 

 cannot be said to be controlled by gravitation, nor can it be 

 purely automatic. 



How are we to explain that remarkable centripetal movement 

 of the pronuclei which always forms the concluding step in the 

 arrangements preparatory to development? As is well known, 

 these bodies are invariably formed at or near the surface of the 

 egg, sometimes near the same pole, and at other times at op- 

 posite poles. Any attraction assumed to exist between them 

 would only be competent to account for their coming together, 

 but would afford no explanation of their centripetal movement. 

 The direct influence of gravitation can no more account for 

 this than for the centrifugal movement of the germinal vesicle. 

 We are driven to the conclusion that the phenomenon is due to 

 the interaction of nuclear and cytoplasmic forces. It is not 

 admissible to assume that either factor is passive, but rather 

 that each acts and reacts upon the other. It is by virtue of 

 this subtle interaction (Wechselwirkung) that the pronuclei 

 ultimately assume a position of equilibrium with respect to the 

 active constituents of the ooplasm. 



But will the attraction which we must assume to exist be- 

 tween the ooplasm and the pronuclei account for all that we 

 know about the behavior of the latter? Or are we under the 

 necessity of assuming still another attraction acting between 

 the pronuclei themselves? The majority of writers would 

 answer unhesitatingly the first question in the- negative, and the 

 second in the affirmative. While I fully concur in this opinion, 

 it seems to me that the reasons generally assigned for it require 

 examination. 



The formation of asters in connection with the pronuclei, and 

 the fact that these bodies approach each other and eventually 

 unite at the centre of the o.^'g, or at some point which, though 

 not the geometric centre, yet represents the virtual centre of 

 their sphere of action, are commonly regarded as proof that 

 they attract each other. If we analyze these facts, we shall 

 find that they afford very little evidence in support of this view. 



The astral displays demonstrate action of some unknown kind 

 between the ooplasm and the pronuclei, but they give no posi- 



