26 



EFFECT OF X IRRADIATION ON THE 

 MOUSE FOETUS 



A. S. Fraser and R. J. Hall 



Commonwealth Scientific and Industrial Research Organization, 

 Animal Genetics Section, Zoology School, University of Sydney 



Irradiation during foetal development can produce permanent modifica- 

 tions of the structure of the resultant organism, which, because they mimic 

 the effects of mutants, have been termed phenocopies. Various authors have 

 shown that the frequency and type of abnormality are closely related to the 

 time of exposure, and this gave rise to the concept of 'sensitive or critical 

 periods', in which each sequence of development was thought to pass through 

 one or more phases when it was sensitive to irradiation. Russell^ in an 

 extensive series of experiments identified the sensitive periods of a number of 

 developmental sequences in the mouse. Most of her researches were con- 

 cerned with fairly macroscopic aspects of structure, and we, therefore, 

 decided to extend these researches, concentrating on any effects on the coat 

 and skin. The primary aim was to compare the sensitive periods of macro- 

 scopic characters, with those for as small a structure as could easily be scored. 

 The facial whiskers are such a structure. They are large, and therefore, 

 easily scored, yet are formed by a very small living structure, the skin 

 follicle'^. The same data are of interest in considering patterns of radio- 

 sensitivity. This is the primary aim of the present paper. 



Pregnant mice, dated by the vaginal plug method, were irradiated with 

 X-rays, generated at 85 kV, at a rate of 43 roentgens per minute. Three 

 doses were used: 250, 300 and 350 roentgens. The mice were scored (/) as 

 dead or alive, (n) as showing external morphological abnormalities, and (Hi) 

 for presence or absence of the three secondary groups of facial \vhiskers. The 

 frequencies of mice, scored at birth, which were dead or alive, normal or 

 abnormal, are given in Table 1. Presence or absence of facial whiskers are 

 not included in these classifications. 



The incidence of phenocopies of external morphology are, from these data, 



Table 1. Frequencies of mice, scored at birth, which were dead or alive, normal or 



abnormal 



Normal Abnormal 



X-ray dose Alive Dead Ali\ e Dead 



248 



