FETAL X-RAY EFFECTS: CEREBRAL CORTEX 37 



diflferences appeared in cortical thickness and brain weights and were gen- 

 erally of the same magnitude as reported for the 300 r series. The values for 

 the half-body group did not differ significantly from controls in any of the 

 parameters studied. 



Discussion 



Rugh (1959) has made the observation that loss of cells in the embryo or 

 fetus due to irradiation may be obscured by the fact that the organism has 

 remarkable powers of integrating the remaining undiflferentiated, undam- 

 aged neurectoderm into a topographically normal but reduced whole or- 

 ganism. Oiu' results appear to furnish considerable suppoi t for this \ iew in 

 reference to cerebral cortex. 



While the curxes illustrating increase in cortical thickness are plotted only 

 tor area 2 i Fig. 5), it is seen on plotting the \alues for the other areas that 

 the pattern of increase in cortical thickness in all radiated and normal series 

 is characteristic for each area. While cortical thickness \aries inversely with 

 radiation dose at the 200 and 300 r lexels in all areas, the cinves illustrating 

 increase in cortical thickness at all dose lexels and in coiitrol series tend to 

 be parallel tor a given cortical area, lliis strong tendency toward parallel 

 growth within a specific area, together with the lack of any truly significant 

 volumetric or density changes, indicates that the surviving cells must possess 

 essentially the same de\elopmental capabilities as the nonirradiated cells ot 

 the control animals. 



The degree to which the orientation and pattern of branching ot cell 

 processes may differ in irradiated and control groups at the dose and rate 

 levels used remains to be determined. The cjuantitatixe technique developed 

 by Sholl (1953) and Eayrs (1955) should be admirably suited to the solu- 

 tion ot this problem. However, it appears clear from our determinations 

 that, at the dose le\els employed, there are no significant differences in cell 

 territory (computed from neuron packing density) in any of the irradiated 

 or control groups. This indicates that, whether or not the cell processes 

 may be altered morphologically, they must, on the average, occupy the same 

 cortical \olume in irradiated groups as in control animals. 



Hicks (1959) has shown that rather specific, predictable nervous system 

 anomalies may be produced with single exposures ot x-irradiation from 

 about 150 to 200 r at any given time in the gestation period after the 9th 

 day. Our results suggest that with fractionated doses between the 10th and 

 17th gestational days it may be possible to produce animals with cerebral 

 cortexes in which definite and predictable cellular deficits, rather than gross 

 anomalies, exist in the absence of any \olumetric. density, or general cyto- 

 logic changes in the cells present. From our obser\ations it appears that the 



