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rence of amitosis is noted and its role in the normal and regulatory 

 growth of many species as indicated in the present paper, it becomes 

 evident that we are not dealing with a phenomenen of secondary im- 

 portance and the need for a working hypothesis is great. In the 

 following paragraphs an attempt is made to formulate such a hypo- 

 thesis. The future will determine whether this hypothesis is correct 

 or not, but I believe that it accords with the known facts. 



To my mind the most characteristic feature in all cases of ami- 

 tosis described is the apparently non-cyclical or orthodromic character 

 of the processes involved, so far as the nuclear contents are concerned. 

 These processes appear to consist essentially in the continued pro- 

 duction of new nuclear material like that already present and without 

 the periodical recurrence of metamorphosis. The act of division itself 

 is very probably a mere physical incident of the increasing volume of 

 substance. 



Mitosis on the other hand is very evidently associated with cycli- 

 cal processes in the nucleus, for alternate departure from and return 

 to certain conditions are characteristic features. 



This apparent distinction cannot at present be reduced with any 

 certainty to terms of physics and chemistry. The visible phenomena 

 seem, however, to indicate that periodical reversal in the direction of 

 reactions or changes of phase occurs in connection with mitosis, while 

 in nuclei dividing amitotically such reversal does not occur, though 

 the processes involved are not necessarily irreversible unless carried 

 beyond a certain point. Attempts at more complete interpretation in 

 these terms are at present unprofitable. 



If these suggestions as to the nature of the processes with which 

 the two forms of division are associated are correct we may expect 

 to find amitosis occurring in regions of rapid growth as well as in 

 regions where active processes of secretion or reserve-formation are 

 concerned : moreover, since orthodromic processes pushed to the extreme 

 must always result in total destruction of the original substances it is 

 not strange that degeneration frequently follows amitosis, but there is 

 no reason for believing that it must always follow, and the facts prove 

 that it does not. The future may show us that amitosis is associated 

 only with certain, not with all orthodromic processes in the nucleus, 

 but however that may be, I believe the recognition of its apparently 

 orthodromic character is of considerable importance. 



Furthermore, according to this view, the absence of forms of divi- 

 sion intermediate between amitosis and mitosis in an organism or 

 tissue where both are occurring need not surprise us, for the two 



