RADIATION IN PRENATAL DEVELOPMENT 887 



This broad division of the gestation period has already been mentioned 

 and the special features of the first phase (preimplantation) have been dis- 

 cussed. Second, it is apparent that critical periods 7 for the induction of 

 almost all abnormalities are short. The 200-r series shows many of 

 them to be restricted to one particular stage (e.g., spina bifida, reduction 

 of the ilium, abnormalities of the basihyal, etc.), and even when they 

 span several stages, one often stands out as the main sensitive one on the 

 basis of incidence (e.g., anterior premaxillary fusion, vertebral jumbling, 

 rib fusion, coloboma). Third, the effective period of disturbance is 

 lengthened by raising the dose, indicating that a certain degree of sensi- 

 tivity exists at stages other than the most critical one. High doses are 

 thus not suitable for the mapping of critical periods except for abnormali- 

 ties which have high thresholds of induction (e.g., deformities in limbs 

 and girdles). The addition of higher dose series is useful, however, not 

 only for conclusions which can be drawn from dosage comparisons (dis- 

 cussed later in Sect. IIIC) but also for the confirmation of location and 

 shape (e.g., unimodal, or bimodal as in tail reduction, etc.) of any given 

 critical period. 



b. Experiments That Concentrate on a Few Stages or on a Group of 

 Abnormalities. In contrast to the experiments discussed so far, Wilson 

 and Karr (1951) confined their X-ray treatment of rat embryos to day 10. 

 Survivors which had received 50, 100, or 200 r (18, 40, and 31 embryos, 

 respectively) were sacrificed 1-5 days later and examined for morpho- 

 logical changes. A small number of embryos in the 100-r group were 

 allowed to come to term and were observed postnatally. All the pre- 

 natal findings of aberrant growth in various organs, as well as damage to 

 the liver (classed by the authors with localized retardations rather than 

 with malformations), are represented in Figs. 13-4 to 13-7. It may be 

 noted that 50 r was ineffective except in the production of two cases of 

 slight microphthalmia. For most changes, incidence with 200 r is higher 

 than with 100 r. The eye was the most consistently affected of all 

 organs, giving 75 per cent abnormalities even with 100 r. Defects of the 

 extremities and liver damage cannot be diagnosed until the thirteenth or 

 fourteenth days, when these organs have reached a stage of differentiation 

 adequate for observation. Percentage incidences are, therefore, based on 

 considerably lower totals (14, 27, and 16 animals, respectively, in the 

 three dose groups) but, as they stand, they approximate eye defects in 

 frequency in the 200-r, though not in the 100-r group. Urogenital mal- 

 formations include (in order of decreasing frequency) incipient horseshoe 



7 The critical period for a given change from normal development may be defined 

 as the developmental moment or interval during which radiation must be applied to 

 produce that change at some specified stage of observation, if the dose of radiation is 

 the lowest one that gives a detectable incidence of that change. The term "critical 

 period" does not necessarily imply that the immediate primordium of the character 

 malformed as a result of irradiation at a certain stage was damaged at that stage. 



