892 RADIATION BIOLOGY 



localized radiation and could thus not be expected to indicate differential 

 sensitivities throughout the body. 



It should be pointed out that several approximations have had to be 

 made to construct Figs. 13-4 to 13-7. They represent work on two differ- 

 ent animals — rat and mouse — which had to be matched according to 

 equivalent developmental stage. In most of the original publications, 

 data are not represented in a manner to indicate critical periods. Many 

 are incomplete: Job does not report totals for calculation of percentage 

 incident, Kaven only occasionally; Kaven's observations were apparently 

 not thorough since he missed limb defects clearly indicated in one of his 

 illustrations; Warkany and Schraffenberger list only wide dose ranges 

 (e.g., 190-900 r) instead of separating results according to dose. It must 

 also be remembered that the thorough work of Wilson and Karr was 

 confined mainly to day 10. 



Despite the various limitations, agreement in most cases is good and 

 some of it is striking. Only some of the comparisons can be discussed 

 here. In the work of three authors, the critical periods for all long-bone 

 defects in the forelimbs seem to be strictly confined, both in mouse and 

 rat, to the stage when the anterior limb buds first become apparent as 

 rudder-shaped outgrowths. The total period for the hind limb is some- 

 what longer and the work of two authors indicates that the tibia is the 

 first bone to be sensitive, the fibula the last. Warkany and Russell agree 

 well, both on Polydactyly and on foot reduction (oligodactyly, syn- 

 dactyly, etc.), but for the latter there is an additional early critical period 

 in the rat (Wilson, Warkany) which has not appeared in the mouse data. 

 The general period sensitive to the induction of eye abnormalities is 

 similar in three experiments. 



There is agreement between authors on apparent bimodalities in 

 critical periods of three characters (ribs, tail, palate). In the case of the 

 ribs, two different abnormalities — fusion and angulation — are obviously 

 represented without overlap in times of sensitivity. For the tail, such 

 distinction cannot be made from the end result, but very probably two 

 different developmental processes are affected (Russell, 1950). All cases 

 of cleft palate resulting from early treatment were found by Russell 

 (1949) to be associated with anterior premaxillary fusion, while those 

 following irradiation during the second period were independent of this, 

 thus probably indicating two entirely different precursor processes for 

 the abnormality. Warkany does not report any such distinction but, 

 since short upper jaw occurs in his material irradiated during the early 

 period, a similar situation may exist. 



Russell found only one instance of situs inversus in the mouse, but this 

 corresponds well in critical stage with the more frequent cases of Wilson. 

 On the other hand, the urogenital malformations described by these two 



