EMBRYONIC DEVELOPMENT 405 



tivity does not run parallel with the increase in division rate. Such a 

 variation as this indicates the presence of other factors. Holthusen (58) 

 and Dognon (37) have obtained results similar to those of Packard by 

 modifying the division rate in Ascaris by temperature changes. 



Still another aspect of the problem of growth rate and sensitivity 

 must be taken into account. Strangeways and Fell, in their tissue-culture 

 work, and Ancel and Vintemberger in their work on the ova of amphibians 

 and birds have demonstrated the necessity in growth-rate studies of 

 discriminating between the growth rate before or at the time of exposure, 

 and the growth rate which would normally occur after exposure. The 

 growth rate and the general activity of cells, which would normally take 

 place during the period immediately following radiation, appear to be of 

 primary importance in determining the degree of injury which results 

 from the radiation. 



For a more complete understanding of this involved problem of growth 

 rate and sensitivity it would be of value to possess accurate quantitative 

 data showing the growth rate in larval stages of some animal whose 

 growth rate could be measured with accuracy, and whose corresponding 

 sensitivity to radiation could also be readily observed. 



SUMMARY 



Exposure to radium or to X-rays results in pronounced and often 

 far-reaching effects on embryonic development in all species of animals 

 which have been investigated. Radiation either of one or of both germ 

 cells prior to fertilization causes abnormal fertilization, and, in some cases, 

 leads to parthenogenetic or androgenetic development. The rate of 

 cleavage is usually retarded by radiation, but under certain circumstances 

 a slight but temporary acceleration of cleavage has been noted. The 

 period of gastrulation and body-axis formation is a critical period for 

 radiated individuals. During gastrulation and the formation of the body 

 axis many abnormalities arise which lead in most cases to the production 

 of monsters and frequently to death of the embryo. 



Observations on the effects of radium and X-rays on avian and 

 mammalian development have been restricted for the most part to the 

 relatively late stages of organogenesis. In both birds and mammals 

 radiation of an embryo affects particularly the most rapidly developing 

 systems, such as the nervous and the vascular systems. Abnormalities 

 in either one or both of these systems are often so serious as to cause 

 death of the radiated individual. Disorders in the reproductive and the 

 nephric systems also often occur after radiation. 



In general, the sensitivity to radium and X-rays during development 

 gradually decreases as the age of an individual increases. There is 

 considerable evidence that decrease in sensitivity with increase in age 

 may be associated with the decrease in growth rate. Although an 



