EMBRYONIC DEVELOPMENT 391 



was an initial acceleration of development which occurred during a short 

 period after radiation. They found, however, that the general effect of 

 radiation was developmental retardation, and that many abnormalities 

 appeared in radiated newts and chicks. Tur (89), who used radium 

 bromide, and Bordier and Galimard (25), who used X-rays, also reported 

 that arrested development resulted from the radiation of chick embryos. 



The work of Perthes (79) is probably the most systematic of the earlier 

 investigations. He exposed the ova of the nematode worm, Ascaris 

 megalocephala, to various dosages of X-rays and to 10 mg. of radium 

 bromide for various lengths of time, and studied the changes which were 

 brought about in the early stages of development. The general result, 

 when the ova were exposed either to X-rays or to radium bromide was a 

 slowing of development. The extent of the developmental retardation 

 Perthes found to be proportional either to the X-ray dosage or to the 

 length of exposure to radium bromide. Moreover, in addition to being 

 much slower, cleavage in the radiated ova was in many cases very irregular 

 and abnormal embryos often resulted. These al)normal embryos were 

 usually irregular masses of cells which possessed low vitality. 



All of the early investigations were important primarily in demon- 

 strating that both radium and X-rays exert a pronounced and often 

 far-reaching effect on embryonic development in such widely separated 

 animals as the echinoderms, the nematodes, the amphibians and the 

 birds. The most commonly reported effect was general developmental 

 retardation, which in some cases might be preceded by an initial accelera- 

 tion of short duration. In connection with the retardation of develop- 

 ment many structural abnormalities appeared. Later work has dealt 

 for the most part with an analysis of the effect of radium and X-rays on 

 particular phases of embryonic development, and with a more critical 

 examination of the abnormalities which are produced. 



MODIFICATIONS IN EARLY STAGES OF EMBRYONIC DEVELOPMENT 



FERTILIZATION 



The effect of radiating one or both of the germ cells previous to fertili- 

 zation has been the subject of considerable investigation. In his studies 

 on amphibian development Bardeen (18, 19, 20) reported that 

 exposure of spermatozoa to X-rays reduced both their motility and their 

 capacity to fertilize ova. Moreover, ova which were fertilized with 

 radiated spermatozoa usually failed to develop beyond the gastrula 

 stage, and the few gastrulae which continued development formed abnor- 

 mal larvae. A few experiments by McGregor (65) gave substantially 

 the same results. According to Bardeen, radiation of ova before fertiliza- 

 tion had little effect on their capacity for fertilization by normal sperma- 

 tozoa. Radiated ova, however, gave rise to a large percentage of 

 abnormal larvae. 



