EFFECT OF X-RAYS ON RATE OF CELL DIVISION. 'Jl 



widely from the normal by merely placing them in a watch glass, 

 and observing them under a microscope. To expose the eggs to 

 X-rays it is only necessary to set the watch glass under the 

 X-ray tube. 



When considering the results of these experiments and the 

 effects obtained on Planorbis eggs it must be kept in mind that 

 these eggs are normally subjected to wide variation in tempera- 

 ture from mild winter temperature to summer heat and they 

 are unusually well protected. The description given by Holmes 

 (P- 375) of the unsegmented eggs of Planorbis trivolvis fits the 

 egg of Planorbis lentus accurately. He describes the egg as 

 embedded in a thick albumen mass within a capsule, which in turn 

 is surrounded by a jelly mass, and the whole including a score of 

 other similar capsules, is covered by a tough enclosing membrane. 

 In making the experiment the entire cluster only is disturbed. 

 Thus there are eliminated from consideration such factors as 

 changes of temperature, of pressure, of oxygen or carbon dioxide 

 content, etc. In fact the eggs were developing under entirely 

 normal conditions, with the exception that the cluster had been 

 freed from its attachment, at the time of exposure. The only 

 factors that this loosening and the subsequent treatment of the 

 eggs could involve, except the factor under experiment, would be 

 a very slight degree of shaking, changes in the direction of the 

 attraction of gravitation, and changes in light intensity; and it 

 is highly improbable that either of these would have much effect 

 upon the rate of development, or upon the finer structure of the 



egg- 



When it is desired to fix the eggs it is only necessary to break 

 the capsule with a needle and tease them out. I have teased 

 them as Holmes recommends into salt solution which contained a 

 little picric acid and then fixed them in Kleinenberg's picro- 

 sulphuric. 



Under proper conditions of illumination, it is usually possible 

 to observe the grosser details of spindle and aster formation in 

 the living egg, and the early cleavages are not difficult to follow. 

 The normal course of cleavage is approximately as described 

 by Holmes ('oo) for Planorbis trivolvis and by Conklin for 

 Crepidula ('97). The cytological details as worked out on the 



