38 BRIZZEE, JACOBS, KHARETCHKO AND SHARP 



cellular deficiencies are generally distributed through all cortical layers and 

 that neurons and neuroglia are equally afTected. If this were not true the 

 neuroglia /neuron index would reveal more consistent differences between 

 control and irradiated animals than were observed. We believe that the 

 high values for neuroglial density and the neuroglia/neuron index in the 300 

 r group in 1-day animals may reflect an increase in microglial elements as a 

 manifestation of the terminal phases of the reparative processes which follow 

 irradiation damage in the cerebral cortex as described by Hicks (1957). 



The use of graded fractionated doses of x-irradiation during the latter half 

 of the gestation period may offer a means for producing a series of animals 

 with predictable, graded cellular cortical deficits. One might raise the objec- 

 tion, however, that fractionated doses administered on several successive 

 days may produce cellular deficits in many or all parts of the neuraxis. This 

 would obxiously decrease the \alue of such preparations for the study of the 

 effects of localized cortical cellular deficits on learning ability, locomotor 

 function, or electrophysiologic phenomena. It seems probable that better 

 localized cellular deficits without gross abnormalities might be produced by 

 fractionated closes given within a more limited time than used in the present 

 study at certain specific days in the gestational period as shown by Hicks in 

 his timetables of radiation malformations. Such specimens should prove 

 particularly useful in the field of experimental psychology, and investigations 

 aimed at exploring this possibility are now in progress in our laboratory. 



It is interesting to note that Hicks (1959) foimd that single doses of 150 

 to 200 r administered on the 16th day of gestation resulted in cortexes which 

 were only one-half as thick as normal and bore little resemblance to the 

 normal laminated 6-layerecl neocortex except in the lateral region. In our 

 preparations, even at total fractionated doses of 300 and 320 r, while the 

 cortical thickness in area 2 is reduced by approximately 309r, the laminated 

 character of the cortex in rats 20 and 50 days of age is well preserved with 

 surviving cells apparently cytologically normal, thus indicating that the sur- 

 vi\ing cells manage to overcome the effects of the daily low-dose radiation 

 exposure to a great extent, even where the cumulati\e dose is rather high. 

 In earlier stages, cell layers are less well de\eloped in many irradiated ani- 

 mals, especially in areas 17 and 41. 



Nurnberger and Gordon (1957) have recently called attention to the in- 

 adequacy of the more commonly used referents, such as wet weight and dry 

 weight, for evaluating chemical and presumably metabolic properties of tis- 

 sues and emphasized the desirability of employing more meaningful refer- 

 ents, such as cell density. We would like to emphasize the desirability of 

 employing not only cell packing density, but additional referents, such as 

 mean and total nuclear \olume, cytoplasmic volume, glial packing density, 

 glia/neiuon index, dendritic territory, cell surface, and other such parame- 



