Regeneration in the Mesencephalon. 181 



The results are briefly these. The removal of the eye and the 

 brain results in the formation across the gap of the wound of a 

 curtain of tissue in all probability derived from the ependymal 

 lining of the neural tube. The ingrowing fibers of the optic nerve 

 from the left eye apparently stimulate the tissue thus formed to 

 regenerate to a considerable extent. At the same time forward 

 growing fibers from lower centers also afford some stimulus for 

 regeneration, as was shown in the case of the primitive pallium of 

 the telencephalon. The tissue thus regenerated is very similar in 

 its organization to that normally found, except that important 

 optic areas are lacking. An analysis of the fiber tracts involved 

 must be deferred until later. 



The removal of the mesencephalon leaving the eye in its nor- 

 mal position results in an almost complete regeneration of the 

 optic lobes. In one larva only a very slight defect in the right 

 mesencephalon distinguishes it from a normal unoperated individ- 

 ual. The process is apparently a much faster one than it is in the 

 case of the olfactory system for the complete regeneration has 

 occurred at the end of some three weeks, while in the case of the 

 cerebral hemispheres complete regeneration did not occur until 

 the end of as many months. This is really not so strange as 

 would seem on the face of it, because, as the writer has shown 

 elsewhere, the optic sense becomes functionally active some time 

 before the olfactory. The early activity of the eye would then 

 result in an early stimulus to regeneration. 



These results show, as in the former experiments, that func- 

 tional activity of the end-organ normally connected with the 

 brain affords the necessary stimulus to regeneration of the part 

 of the brain removed. 



103 (1167) 



Conduction, excitability and rhythm-forming power of the atrio- 

 ventricular connection in the turtle. 



By Henry Laurens. 



[From the Osborn Zoological Laboratory , Yale University.] 



As in the heart of the turtle Clemmys lutaria and of the lizards 

 Lacerta viridis and agilis (Laurens 1 ) the right and left parts of the 



'Laurens, PJluger's Archiv, 1913, 150. p. 139. 



