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most of the embryo contained at least ten times as many nuclei in 

 each section as the lateral regions. The difference in number is 

 probably even greater but these results will serve for present pur- 

 poses. Now the number of nuclei in the middle region including 

 the somites and the neural tube is undoubtedly increasing more rapidly 

 than that in the lateral regions, yet with at least ten times as many 

 nuclei per section it shows only twice as many mitoses. If we assume 

 that the nuclei in both regions are multiplying at the same rate and 

 that all divisions are mitotic then each mitosis in the middle regions 

 must occur in one fifth of the time required for a mitosis in the 

 lateral regions. But there is absolutely no reason to believe that such 

 a difference in speed of the mitotic process exists in the two regions. 

 Moreover, nuclear multiplication in the middle region is undoubtedly 

 more rapid than in the lateral regions. If the count of mitoses ex- 

 tends over a somewhat larger area of the lateral extra-embryonic 

 region on each side the number of mitoses in the region lateral to 

 the lateral border of the somites equals the number between the lateral 

 borders. In other words, there is no doubt that at this stage the 

 number of mitoses in the regions lateral to the somites and extending 

 beyond the limits of the embryo proper is much greater in proportion 

 to the number of nuclei than in the middle region. I have rarely 

 seen anything that looks like amitosis in this lateral region at this 

 stage. These facts seem to indicate that a considerable part of the 

 nuclear multiplication in the embryo is not the result of mitosis. 



The nuclei shown in Figure 11, all of them from the middle region 

 afford positive evidence in favor of the occurrence of amitosis. It 

 seems probable that counts of mitoses in embryonic regions whose 

 rate of nuclear multiplication can be compared will afford some inter- 

 esting data regarding the significance of mitosis. Some of my col- 

 leagues have informed me in answer to inquiry that they have been 

 unable to discover a relation between the number of mitoses and the 

 rapidity of nuclear multiplication in certain embryonic tissues. Further- 

 more, during the past few years numerous cases have come to my 

 attention in the literature of embryology and histogenesis in which the 

 rarity or absence of mitosis was noted in regions where rapid nuclear 

 multiplication was evidently taking place. 



Discussion. 



I. The present State of our Knowledge regarding 



Amitosis. 

 While I am quite aware of the somewhat fragmentary nature of 

 the preceding observations, I believe that they indicate that amitosis 



