EFFECT OF X-RAYS ON RATE OF CELL DIVISION. 



they gradually get slower and slower until they are developing 

 less rapidly than the normal eggs. Thus the last result is a 

 retardation of growth and cell division. 



The following experiment on the eggs of Physa gives additional 

 evidence in support of the conclusions just stated and also suggests 

 certain other effects of the rays. There are three sets of data 

 given, and the eggs used were all from the same cluster. All 

 were in the same stage at the beginning of the experiment. In 

 the first column is shown the rate of development of the control ; 

 in the second, are the stages for corresponding times of the eggs 

 which were exposed five minutes to the X-rays; and in the third 

 are similar observations for eggs exposed ten minutes. It will 

 be seen that in the shorter exposure the divisions are going more 

 rapidly than the control, but at the end of the experiment they 



+ 18 min. ist cleavage furrow Blastomeres flat- One egg divided, 



beginning to di- tened against each others show no ex- 

 vide the eggs. other. ternal signs. 



+20 min. 



ist division com- No record. 



Furrow beginning to 



were getting slower. These radiations were made with an 

 apparatus carrying a stronger current and giving off more intense 

 rays than hithertofore. To that condition is without doubt due 

 the effects produced by the stronger radiation. In this last case 



