BRAIN REGENERATION IN AMBLYSTOMA 209 



previous exiDerinient. The healing of the wound brings the 

 nasal placode back to approximately its normal position though 

 a slight shifting anteriorly has been noted. Five days after 

 the operation the curtain is composed of a single layer of cells 

 (fig. 1). Two days later this single layer of cells has become 

 converted into a thickened mass of colmnnar cells that are evi- 

 dently of ependymal origin (fg. 2). By the time the twelfth 

 day is reached the olfactory- nerve has established a connection 

 with the curtain, which has in the mean time increased con- 

 siderably in size. The establishment of the nervous connection 

 betw(^en the nasal placode and the telencephalon takes ]ilace 

 iu the noi-mal larva about the tenth day, only a slight delay 

 in this union occumng as a result of the operation. In the 

 unoperated larvae the nasal placode lies in close apposition to 

 the thick wall of the telencei)halon, the olfactory fibers growhig 

 into the hemisphere at the ])ohit of contact. In the oi)erated 

 foil lis the nasal anlage lies close to the newly formed curtahi. 

 which as nulicated al)ove shows a slight thickening not seen in 

 the operated larvae possessing no nasal placode. This thicken- 

 ing anticipates the ingrowth of the olfactoiy nerve by several 

 days. It is not possible at lliis tiuu^ to give any explanation 

 of this thickening, though it is conceivable that the presence of 

 the olfactory anlage stimulates the ependymal cells to grow and 

 divide through some mechanical or chemical factor. 



Two days after the ingrowth of the olfactory ner\e the new 

 telencephalon, as it must now be called, has reached quite an 

 advanced stage of de\'eloi)ment. During these two days the 

 thickened curtain has l)een differentiated into a small but never- 

 theless typical telencei^halon. Neuroblasts are present and 

 a ventricle has ap]ieared. Outwardly the form has changed. 

 A forward growth has converted the thick flat plate of cells 

 into an ovoid mass of neuroblasts and spongioblasts. The 

 ingrowth of the olfactory nerve on the lateral aspect of the mass 

 is accompanied by the appearance of the large lateral forebrain 

 tract (fig. 4). Almost sinuiltaneously there appears from the 

 dorsal aspect of the olfactory bulb the tractus olfactorius dorso- 

 lateralis and from the ventral aspect the tractus olfactorius 



