EFFECT OF X-RAYS ON RATE OF CELL DIVISION. 77 



Similarly at the end of the maturation divisions before the 

 fusion of the male and female pronuclei no marked stimulation 

 could be produced by exposure to the X-rays. 



Contrasting with the condition just described is that found in 

 the cleavage mitosis. If one may speak of the cell in the resting 

 nucleus stage as being in a state of equilibrium or stability with 

 respect to its capacity to respond to stimulation, we may like- 

 wise say that during a mitosis it is relatively unstable with respect 

 to this particular character; 1 and there appears to be a certain 

 time during the course of spindle formation when the capacity 

 of the cell to respond is greatest. During the period from the 

 definite formation of the spindle to the metaphase or anaphase, 

 response to X-ray stimulation is easiest to obtain. Similar 

 results have been reported for experiments with various means of 

 stimulating the egg. Conklin ('13) found much clear evidence 

 to prove that "the early stages of cleavage are more sensitive to 

 environmental changes than later ones." 



1 (7) The eggs of this experiment had just completed the formation of the first 

 polar body and the second maturation spindle was beginning to form when they 

 were exposed to X-rays for ten minutes. At the end of the exposure the second 

 division seemed to be entirely complete. The first cleavage division was accom- 

 plished in the next fifty minutes. The succeeding divisions occupied more time, 

 for one hour and thirty minutes elapsed before the completion of the second cleavage, 

 and a like period of time passed before there was any sign of a micromere division. 

 Later divisions were even more slow. The control was far ahead in its divisions, 

 having reached a stage where the blastomeres could no longer be counted accurately 

 in the living condition. 



(8) The eggs of this experiment were exposed ten minutes during early stages 

 of the second cleavage spindle. When the exposure was ended it was found that 

 in many cases the division was completed, and in all it was well along. The third 

 division occurred in thirty five minutes more. Owing to an accident, further data 

 on this set of eggs were not obtainable. 



(9) The eggs in this set were secured during the spindle formation of the first 

 maturation division. The exposure lasted six minutes; at its conclusion, the cyto- 

 plasm could be seen collected largely at one pole of the egg, and in some cases a 

 polar body elevation was apparently beginning to form. In half an hour both 

 maturation divisions were completed. One hour and twenty minutes were con- 

 sumed in the first cleavage division and one hour and fifty minutes more in the 

 second division. Two hours later no advance was noted. 



(10) Eggs in the late prophase of the first cleavage were exposed three minutes, 

 at the end of which time they were found to have completed the division, and in 

 five minutes more the blastomeres had flattened against each other with the peculiar 

 lenticular cavity between them. In thirty minutes the second cleavage had occured 

 and in two hours and a half the fourth had been accomplished. 



