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BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



second table the photomechanical changes were carried out completely^ thus 

 demonstrating that the brain is not in any way essential to these changes. 

 Why this completeness was not seen in other cases I am unable to state 

 positivelj^ though I believe it was owing to the changes that gradually 

 appear in the tissue of the stalk after its severance from the body of the 

 animal. When an optic stalk is excised, the blood in it soon coagulates 

 and other alterations doubtless start up, which tinally result in the com- 

 plete death of the tissues of the stalk. It is these alterations. I believe, 

 that overtake and bring to a standstill the slowly progressing photome- 

 chanical movements. But, whatever may be the true explanation of the 

 incompleteness of the changes in excised stalks, the general conclusion 

 remains unaffected, that in Palsemonetes the brain is not essential to the 

 photomechanical changes in the retina. 



This conclusion has an important bearing on the question of the sym- 

 pathetic relations of the two retinas in a given animal. Since the two 

 retinas are nervously connected only through the brain, and since the 

 retinas are not influenced from the brain, it follows that the two retinas 

 cannot be sympathetically related, a conclusion to which observations 

 .already recorded have likewise pointed. 



If the photomechanical changes are not dependent in any degree on 

 the brain, it may still be asked whether they are not influenced by the 

 optic ganglia. To answer this question, I carried out on excised retinas 

 a series of experiments similar to those just described for the optic 

 stalks. It is much more difficult to sepai'ate the retina from the optic 

 ganglia than it is to separate the optic stalk from the brain, but with 

 careful manipulation it can be done, and the following tables give the 

 results of experiments carried out upon such retinas. 



Four Rigid Retinas in Dark Condition cut off and placed in the 

 Light about two Hours. 



The four left retinas kept in the dark as checks on the experiment 

 exhibited the noi'mal condition for the dark. 



