356 A. RICHARDS. 



is one that is familiar to experimental embryologists, having been 

 demonstrated with many eggs and many agencies. 



Bohn found that an exposure to radium of forty minutes ac- 

 celerated segmentation in eggs of the sea urchin, although a longer 

 exposure retarded it. 



The use of x-rays agrees in most respects with that of radium. 

 Gilman and Baetjer exposed hen's eggs for ten minutes daily to 

 x-rays. During the first thirty-six hours the development was 

 accelerated. Then there followed a retardation during which the 

 development was greatly altered as well as checked. Comparable 

 results were obtained by these same investigators working on the 

 eggs of Aniblystoma. Exposures of fifteen minutes daily first 

 produced a period of acceleration which lasted up to ten days in 

 some embryos, but at the end of the fourth day abnormalities 

 began to manifest themselves. 



While investigating the effect of x-radiation on Planorbis eggs 

 the writer found convincing evidence of their ability to accelerate 

 cleavage. The eggs normally require from fifty-five minutes to 

 two hours to complete a division (up to the 24-cell stage). The 

 first effect of exposure of not to exceed ten minutes, if made 

 during the formation of the mitotic spindle, is to accelerate the 

 division of the egg. Even a very short stimulation will produce 

 this phase of acceleration, which is then followed by a phase of 

 depression ; the end result is to retard greatly the development of 

 the egg. The acceleration of the rate of cleavage in Planorbis as 

 a result of exposure to x-radiation is the most extreme response 

 of this character with which the writer is acquainted. ' This 

 effect was first obtained after eggs had been exposed ten minutes, 

 when it was noticed that divisions had actually been completed in 

 cells where only a spindle was to be seen at the time the exposure 

 began that is, during an exposure of ten minutes there had been 

 accomplished a complete process which never under normal condi- 

 tions had been observed in this form to occur in much less than an 

 hour. I have repeated this observation from January to June on 

 many experiments and have obtained the result without variation. 

 Whenever an egg of Planorbis in any cleavage up to the sixth, 

 farther than which it is not practical to carry on observations on 

 the living egg, is exposed to x-rays any mitosis which may have 



