RADIATION IN PRENATAL DEVELOPMENT 907 



on day 8H postcohception increases the presacral number to 27 in 100 per 

 cent of BalbC strain animals, in only 3 per cent of (C57 X NB)Fi's, and 

 in per cent of 129 strain mice. However, control results show the 

 BalbC and 129 distributions to be situated across the 2 %q thresholds, 

 respectively, while the (C57 X NB)Fi distribution crosses neither thresh- 

 old. The positions are such that equal radiation-induced shifts in 

 the mean, on the scale of underlying processes, would account for the 

 results. In the course of this experiment it also became apparent that 

 for the changes studied, i.e., quantitative characters in which even slight 

 shifts are observable, a dose as low as 25 r may be shown to have an 

 effect on prenatal development. 



Although the commonest result of an increase in dose is increased 

 incidence and degree of abnormality, there are a few exceptions which 

 point to the possible existence of interesting developmental pathways. 

 In the case of microphthalmia, for instance, the results of both Russell 

 (1950) and Wilson and Karr (1951) indicate a lowering of incidence as the 

 dose is raised. This can be explained by some other abnormality "com- 

 peting" with microphthalmia at the higher dose but produced only to a 

 slight extent or not at all at the lower. — Certain primordia are apparently 

 capable of only one type of response since degree does not change with 

 dose level even though incidence does. Examples are Polydactyly, and 

 hydro-ureter (Russell, 1950). 



D. COMPARISON WITH OTHER AGENTS AFFECTING DEVELOPMENT 



Radiation-induced developmental abnormalities may be compared with 

 those produced by other agents (Haskins, 1948; Gillman et al, 1948; 

 Hamburgh, 1952; Waddington and Carter, 1952; Fraser and Fainstat, 

 1951a; Kalter and Fraser, 1952; see also review by Fraser and Fainstat, 

 1951b). Haskins' (1948) results for nitrogen mustard treatment between 

 days 13 and 16 of rat gestation are similar to (but less extensive than) 

 those obtained by Warkany and Schraffenberger following irradiation at 

 the same stages. On the other hand, Waddington and Carter (1952) point 

 out that trypan blue (used also by Gillman et al. and by Hamburgh) pro- 

 duces a smaller variety of abnormalities than found by Russell for irradia- 

 tion at the comparable stage, and the same seems true of the action of 

 cortisone (Fraser and Fainstat) and of 17-hydroxycorticosterone (Kalter 

 and Fraser). This may well be due to chemical affinity of these deleteri- 

 ous agents for certain primordia. It is also of interest that the high 

 proportion of tail abnormalities found by Waddington and Carter and 

 by Hamburgh for trypan blue injection on day 7 points to the persistence 

 of the deleterious action until somewhat later stages, at which X-ray 

 results have shown the tail to become considerably more sensitive. On 

 the other hand, cleft palate can be produced by the injections of cortisone 

 at stages following closure of the nasomaxillary fissure. Fraser and 



