168 SCIENTIST 



newly made microscopic bubble chamber, four from ani- 

 mals that were in the process of learning a conditioned 

 reflex and four that had received the same stimulation but 

 had failed to learn. All eight animals had received injections 

 of equal amount of isotope labeled purine. The idea was 

 to see if the learners showed an increased synthesis of 

 nucleoprotein by fixing more of the radioactive isotope. The 

 design had some bugs in it, and the experiment wouldn't 

 be at all conclusive, even if it turned out as they hoped. 

 Nevertheless a successful experiment would at least suggest 

 that they were on the right track and justify putting in some 

 additional refinements and controls which they had already 

 thought of. The slides were marked in code by the girl who 

 had made the thin sections of nerve cell they were going 

 to look at. She was the only person in the lab who knew 

 which slides came from the learners and which from the 

 dull animals. 



By the time they had finished the first four preparations, 

 it was clear that one produced about half again as many 

 tracks in a given time as had each of the other three. Nick's 

 hand shook a little as he put the fifth sUde on the stage and 

 started the servos which controlled the humidity and pres- 

 sure. The temperature was taken care of by the thermostat 

 which controlled the small room they were in. After ten 

 minutes he said, "Gee, Bill, it does look as if this one had 

 50 percent more too. And look here, do you see what I see? 

 The extra ones all seem to be coming from the base of one 

 dendrite about 30 degrees from the axon hillock." Bill, who 

 had forgotten most of his neurohistology, took a look, and 

 after Nick had pointed out the landmarks, he thought he 

 could see the difference too. The rest of the morning was 

 spent on surveying the other slides and rechecking the first 

 one to see if it too showed a clustering of counts near one 

 dendrite. It did, but when they had added everything up. 



