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forms of division are the expression of different processes which are 

 not necessarily connected by processes of intermediate character; 

 moreover, morphological phenomena do not necessarily follow a course 

 parallel to physico-chemical changes. On the other hand the occur- 

 rence of intermediate forms is not impossible. Simultaneous occurrence 

 of both cyclical and orthodromic processes with one or the other or 

 neither predominating might conceivably be accompauied by morpho- 

 logical phenomena intermediate in type between the two extremes. 

 Possibly some of the forms of division regarded as intermediate are 

 of this nature. As regards the cases of apparently intermediate 

 character observed under experimental conditions no definite conclusions 

 can be reached at present. 



In several of the cases described in this paper amitosis and 

 mitosis occur simultaneously in the same tissues or regions. On the 

 basis of the suggestions made above this simultaneous occurrence is 

 readily explained by diÖ'erences in conditions in the individual nuclei. 

 Moreover, in several cases I have noted that in growing tissues where 

 nuclei of different size are present mitosis seems to occur more fre- 

 quently in larger nuclei surrounded by considerable undiiferentiated 

 cytoplasm, while amitosis is more characteristic of the smaller nuclei 

 with scanty cytoplasm. Frequently also mitosis is confined more or 

 less closely to certain regions of a growing organ or part, while ami- 

 tosis occurs in other regions. For example in the neural tube of 

 Squalus and chick embryos mitosis rarely occurs except in the nuclei 

 adjoining the neural canal while I have found nuclear conditions which I 

 cannot interpret except as amitosis in other regions of the tube. 

 Fig. 9, I and II (Squalus), and Fig. 12, II — VI (chick), show a few of 

 the cases observed. In regulation in Planaria mitosis is not found in 

 the new tissue but does occur in those parts of the old tissue which 

 are undergoing redifferentiation in consequence of the operation. 



I am inclined to believe that mitosis instead of being the sole 

 criterion of nuclear multiplication in normal growth may in many 

 cases be characteristic of those regions where nuclear multiplication 

 is comparatively slow, while regions where the formation of new nuclei 

 is rapid may show only or chiefly amitosis. The relative frequency of 

 mitosis in the lateral and middle regions of chick embryos, to which 

 attention was called above, suggests that this may be the case, espe- 

 cially since apparent amitosis were frequently found iu the middle 

 region and but rarely outside. The localization of mitosis in the 

 neural tube and other organs may be of the same character. During 

 regulation in Planaria the region where amitosis occurs is the region 



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