EFFECT OF X-RAYS ON RATE OF CELL DIVISION. 8 1 



present some one may often be found in nearly every stage of 

 the cleavage division in progress at that time. Reference is here 

 made particularly to the first or second cleavage. But it is very 

 unusual to find eggs in the cluster in f\vo clearly distinct cleavages 

 at once. If a cluster with various stages represented is ex- 

 posed to the rays, the effect as observed at the end of the ex- 

 posure is to equalize the progress by hastening all of the eggs, 

 except any which might not have begun the division at all, to 

 the completion of the mitosis and into the resting stage. 1 That 

 is, if the exposure found some of the eggs of a cluster in an early 

 stage of mitosis and others in a later, it would, by inducing the 

 acceleration in each individual bring practically all into the 

 resting stage at the close of the radiation. 



It cannot be affirmed, however, from my observations that 

 subsequent divisions of such a cluster as that just described would 

 of necessity occur exactly at the same time. 



This observation on a living egg that the divisions are greatly 

 stimulated by the X-rays goes to explain the observed fact that 

 in fixed eggs which have been exposed, mitoses are not so easy 

 to find as in eggs which have not been exposed. 



The phase of acceleration does not last long in these cells but 

 passes off at the end of the first division or perhaps the second 

 after the exposure. Following it there sets in without further 

 exposure a phase of depression, during which the rate of cell 

 division becomes slower and slower. 2 The eggs' activity as 

 regards cell division is markedly inhibited. This invariably 

 occurs, although the extent and nature of the inhibition or 

 depression may not be exactly the same in all cases. This de- 

 pression may amount to a complete stopping of cell division, thus 

 terminating the experiment; or often observation has been inter- 

 rupted that the eggs might be fixed for cytological examination. 



The depression phase occurs without regard to the stage of 

 the development of the egg at which the exposure took place and, 



1 Compare experiment No. 14. (20) The eggs of the cluster were going through 

 the first cleavage division but had not all progressed equally in the division. The 

 exposure lasted twenty seconds, and the eggs had nearly all completed the division 

 at the end of it. Forty minutes later the blastomeres had flattened against each 

 other and an early spindle was to be seen. 



2 Compare experiments (2), (7), and (15). 



