706 TEXTBOOK OF ZOOLOGY 



In all biological experiments the amount of radiation applied is 

 very important. It has been found that a very slight dosage may 

 be temporarily accelerative, a moderate dosage may have no visible 

 effect, a heavier dose may have a temporary inhibiting effect, and 

 a very heavy dose will result in permanent injury. There is usually 

 a lapse of some time following irradiation before the effects appear ; 

 and it has been found that, as the dosage is increased, this latent 

 period becomes shorter. There are several physical factors which 

 affect the amount of radiation produced ; these are the voltage, the 

 current, the type of target, the distance from the target to the irra- 



Fig. 372. — Effect of temperature during the irradiation upon the amount of 

 injury produced. These rats are now two weeks of age. They were irradiated 

 when one day of age. Each rat was given a dosage of 2,010 roentgens to a sman 

 region of the back, indicated by the arrow. The radiation inhibits hair forma- 

 tion and it can be seen that this injury is much greater in tlie animals irradiated 

 at the higher temperatures. 



diated object, the filtration, the back scatter, and the exposure time. 

 Radiological experiments have become much more accurate and 

 dependable with the advent of the dosimeter. Such a device meas- 

 ures the amount of ionization in air and records this change on a 

 scale calibrated to read in roentgens. This mechanical measure 

 allows the operator to determine the intensity and the total dose 

 delivered. 



Roentgen radiation has proved to be a very valuable tool in the 

 study of experimental embryology. It has been found that the first 

 cleavage is retarded by appropriate exposures. It has been demon- 

 strated that development can be stopped by heavy dosages. It has 



