654 SYLVAN J. KAPLAN 



procedures which will obtain positive differences in groups treated with 

 conditions given in the present experiments. 



The maze did provide evidence of differences in the learning ability of 

 the various groups. It is important to establish the validity of this instnament 

 as a measure of sensory, motor, and central nervous system functions. This 

 has not been done to any degree of exactness. Lashley (1929) has shown 

 that this maze will detect cortical and subcortical insult in terms of deficient 

 rates of learning, and evidence is available to establish that it is also sensitive 

 to central and peripheral visual defects. Much work is left to be done to 

 answer the question of precisely what this maze does test. 



The maze data provide the principal information for the present discus- 

 sion. They must be viewed with some caution, since they represent the 

 performance of very few subjects per experimental group. Likewise, general- 

 izations regarding the precise physiologic and anatomic correlates of the 

 behavior observed can be made only with extreme conservatism. The gen- 

 eralizations regarding indication of radiosensitivity of the embryonic tissue 

 at the various periods of gestation and to various dose levels of irradiation 

 can be stated with some confidence. It would seem fairly safe to presume 

 that deficit in learning ability is highly correlated with the amount of intact 

 tissue responsible for effective learning procedure. 



Fetal Age at Time of Exposure and Dose Magnitude 



When a relatively large dose (e.g., 200 r) was given on the 1.5 day of 

 gestation, it did not seem to impair tissues necessary for the cognitive 

 processes required for maze learning. The group receiving these conditions 

 did not differ significantly from the normal controls. This finding is con- 

 sistent with the statement of Hicks (1953) that the nervous system at this 

 period of gestation is in its most primitive stage and is relatively insensitive 

 to irradiation. 



On the other hand, relatively low doses (25 r, 50 r) were associated 

 with deficient maze learning at 2.5, 6.5, and 7.5 days during gestation. 

 This information is inconsistent with the suggestion by Hicks that irradiation 

 insensitivity prevails throughout the 1st week of embryonic life and supports 

 the findings by Rugh et al. (Rugh, 1959a, b; Rugh and Grupp, 1959a, b) 

 that neural tissue, at least in the embryonic mouse is, extremely reactive 

 even to very low doses from the 1st day of gestation on. 



This conflict of data can be clarified with some assumptions based on 

 further examination of the current findings. Hicks and Rugh have reported 

 that the period around the 9th day of gestation produces dramatic head 

 malformations (anencephaly) . These led to the emphasis on a study of the 

 8.5 day irradiations in the present work. Yet, with the pronounced indi- 



