EFFECT OF X-RAYS ON RATE OF CELL DIVISION. 83 



the stimulation. This, however, is the condition one would 

 expect in view of the fact that a complete mitotic cycle has not 

 elapsed since the exposure, for there is of course no resting ves- 

 cicular nucleus stage between the maturation divisions. It is, 

 therefore, not a real exception, but on the contrary is quite in line 

 with the other observations. 



Eggs which have been exposed to X-rays and have passed into 

 the depression phase may again be stimulated by a new exposure 

 to the rays. 1 These exposures may both be as short as twenty 

 seconds and they are both subject to the conditions previously 

 described, but there is no question as to whether the effect will 

 be produced. However, the new phase of acceleration is not 

 so great nor so clearly marked as the first, while the phase of 

 depression comes on sooner and takes place more rapidly than in 

 the case of the first exposure. 



The relation which the phase of depression bears to the 

 normal development may be illustrated graphically. The 

 following data, plotted in Fig. 2, are from a representative ex- 

 periment. 



The control or normal is used as a base line and the variations 

 of the exposed eggs from the control are plotted with respect to 

 it. The curve, of course, does not show the phase of acceleration. 

 It indicates what has already been set forth, that owing to the 

 depression the time required for a division gradually lengthens 

 relatively until it is equal to the normal rate, and then falls 

 below. The divisions get relatively slower and slower. 



The phase of depression is of sufficient interest to warrant 

 more extended study. To what extent it occurs and how far it 

 may be carried with recovery of the eggs are questions to which 

 I cannot now give a satisfactory answer. 



1 Compare experiments (18) and (19). 



