204 H. SAXTON BURR 



the brain would then be due to the fact that the entire substance 

 of the hemisphere was not removed, enough being left to carry 

 the regeneration through to a considerable degree of complete- 

 ness. A condition similar to this has been shown by Harrison 

 to exist in the extirpation of limb buds in Amblystoma (Harri- 

 son '15). 



So far as it has been possible to ascertain, no attempt has 

 previously been made to control the stimulus afforded by the 

 functional activity of the end organ normally connected with 

 the part of the central nervous system removed. Extirpation 

 of a portion of the brain has invariably carried with it the re- 

 moval of the end organ. It is a little difficult then to see why 

 a part should regenerate when its activity has ceased owing to 

 the removal of the end organ. 



In order to obtain some answer to the above question the 

 following experiments were performed on Amblystoma larvae. 

 Two series of operations were undertaken on embryos possess- 

 ing neither peripheral nerves nor a circulatory system. In 

 the first of these the right cerebral hemisphere and the right 

 nasal placode were extirpated, the cut that severed the hemi- 

 sphere from its connections passing directly in front of the optic 

 stalk. In the second series the right telencephalon was removed 

 but the right nasal placode was left in position. This was ac- 

 complished by turning back the flap of skin containing the pla- 

 code and removing the underlying forebrain. The flap was 

 then returned to its former position and held in place until the 

 wound had healed. Great care was taken to remove all of the 

 cells of the cerebral hemisphere in all the operations. 



The above experiments subject the brain tissue left by the 

 extirpation of the hemisphere to two conditions. In the first 

 series of operations the nervous tissue is left to regenerate with- 

 out the possibility of any stimulus from the end organ that is 

 normally connected with it. In the second series the end organ, 

 the nasal placode, is left in its normal position and may there- 

 fore act as a stimulus to the nervous tissue (Burr '16). 



In the first series of experiments in which the right hemisphere 

 and the right nasal placode were removed the wound usually 



