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DISCUSSION 



Heidelberger: At the McArdle Laboratory at the University of 

 Wisconsin, Price and Laird have obtained results in regenerating rat 

 liver almost exactly parallel to yours. They showed by cell fractiona- 

 tions and chemical analyses that the amount of DNA doubles just before 

 the maximum rate of mitosis, so that the plant and animal pictures 

 seem to be very similar. 



Howard: Yes, this comes out also in recent work of Swift using 

 photometric measurement of Feulgen-positive material in the nucleus 

 both in plant and animal cells. In this work the conclusion is inescapable 

 that the synthesis must be during the resting stage, and possibly also 

 during the first part of the resting stage in the case of the plant cells, 

 with an interval between the end of the synthesis and the beginning 

 of division. 



Leblond: This work is very important from both the practical and 

 theoretical points of view. The results raise some doubt regarding 

 contemporary theories of nucleic acid synthesis (Caspersson and others). 

 Even the interpretation of theories of the origin of cancer may have to 

 be revised. 



I would like to ask a question similar to that of Dr. Heidelberger, 

 since in 1948 I published with Dr. Stevens a paper in which we showed 

 the entry of phosphorus into animal cell nuclei. The reaction of 

 individual nuclei was observed in the intestinal epithelium. By sacrificing 

 the animals at various times after injection we could see the nuclei 

 climbing along the villi of the intestine and gradually moving from the 

 crypts of Lieberkiihn up to the villi of the intestine. We calculated 

 in three different ways the time taken for the nuclei to ascend the villi. 

 First, on the basis of mitotic counts after colchicine, it was estimated 

 that in the rat the rate of renewal of the intestinal epithelium was of 

 the order of 1 • 5 days. The second method was similar to that of 

 Hevesy, estimating the entry of radio-phosphorus in the deoxyribo- 

 nucleoprotein. By that method we obtained a figure that was a little 

 over 2 days. The third method consisted in estimating on ^^p auto- 

 radiographs the speed at which nuclei climbed to reach the tip of the 

 villi, and the figure that was obtained was also over 2 days. We 

 thought that the methods were somewhat rough and maybe the dis- 

 crepancy between mitotic counts (1-5 day) and ^ap methods (over 2 

 days) was due to error variations of one kind or another, but the work 

 of Dr. Howard would suggest that possibly it may be a true discrepancy. 

 Perhaps the cells moving out of the villi soon after the injection are not 



