EMBRYONIC DEVELOPMENT 



395 



takes place. Such a result indicates that recovery of the egg from the 

 effects of radiation may begin as soon as the radiation begins, with the result 

 that the delay in cleavage dates not from the end of the radiation, but 

 rather from the beginning of the radiation. Some of Henshaw's results 

 are shown in Fig. 2, in which the relation of cleavage time to the length of 

 exposure and to the minutes elapsing between radiation and insemination 

 is demonstrated graphically. For correlation with these data it would 

 be of value to know the nuclear and cytoplasmic changes taking place 

 within the egg, particularly from the standpoint of the earlier fertilization 



20 



40 60 80 100 120 140 ^0 180 WT 

 Minu+es After Irradiation Began when Eggs were Inseminated 



Fig. 2. — The effect of various doses of X-rays in retarding the first cleavage of Arbacia 

 eggs. The ordinate represents the retardation of the first cleavage expressed in minutes, 

 and the abscissa represents the minutes after irradiation began when the eggs were 

 inseminated. (From Henshaw, 45.) 



studies of G. Hertwig and of Packard which have been discussed above. 

 Although the data of Henshaw deal only with the retardation of the 

 first cleavage, undoubtedly later changes are similarly affected. Differ- 

 ences arise, however, with regard to the exact moment at which cleaving 

 eggs are radiated. Indeed, it has been shown that when eggs are radiated 

 with the proper dosage at the proper time, acceleration instead of retarda- 

 tion of cleavage may result. Thus, Packard (72) found in the case of 

 Arbacia that a brief but intense application of radium, during the period 

 when the germ nuclei were approaching, accelerated the rate of division. 

 Later exposures, which were made during the formation of the first 

 cleavage spindle, accelerated the rate of cleavage to a greater or less 

 degree, depending on the phase of mitosis at the time of exposure. Such 

 results as these occur, however, only after brief radiation. Longer radia- 

 tion always retards cleavage. Results similar to those of Packard have 

 been obtained through the use of X-rays by Richards (81) on the cleavage 

 of Planorbis eggs and by Richards and Good (83) on Cumingia eggs. Hoff- 

 mann (57) has also reported an accelerated cleavage in frog eggs following 



