EFFECT OF X-RAYS ON RATE OF CELL DIVISION. 69 



obtainable at this laboratory, it was determined to use X-rays 

 rather than radium. No difficulty has been experienced in get- 

 ting results with the X-rays. 



The work which is here reported was carried on at the University 

 of Texas and at the laboratory of the U. S. Bureau of Fisheries 

 at Woods Hole, Mass. The Bureau of Fisheries has kindly 

 given permission for the publication of results obtained at its 

 laboratory. 



For the use of the X-ray machine, which he kindly loaned me, 

 and for his assistance in various ways during the early course 

 of these experiments, I am indebted to Mr. Oliver Brush, of 

 Austin, Texas. 



The snails used were identified for me as Planorbis lentus Say 

 by Dr. W. H. Ball, of the Smithsonian Institution, to whom I 

 wish to express my obligation. They were secured from Waller 

 Creek, a small stream near the University of Texas. 



These experiments were conducted during the early part of the 

 year 1913. After the results had been studied and written up, it 

 appeared wise to delay publication until another breeding season 

 could furnish new material and further experiments could be 

 carried on in order to extend the observations and perhaps give 

 rise to broader conclusions. During December, 1913, however, 

 Texas was visited by one of the most severe floods in its history 

 and the streams were cleanly scoured out. Conditions of vegeta- 

 tion also were greatly changed. As a result, where formerly 

 Planorbis had been found most numerously during the spring 

 months, there are now only a very few scattered specimens. 

 Furthermore, much difficulty has been experienced in getting 

 these specimens to produce eggs. For these reasons I have been 

 unable to renew the experiments on Planorbis. Other fresh 

 water snails suffered largely the same fate during the flood, but 

 their pointed shape enabled more of them to maintain themselves 

 against it, and I have been able, therefore, to study somewhat the 

 effects of the rays on Physa halei. As these eggs are less suitable 

 for detailed study in the living condition owing to the thickness 

 of their gelatinous covering, the statements made in this account 

 apply chiefly to the eggs of Planorbis. In general, the behavior 

 under exposure of the eggs of these two species has not been 



