The Nervous System of Amblv stoma. 



475 



ml ,rnt>J 

 mi\ mhz'. ^ 



mf .miPJ 



Fig. 12. Side views of the neural tubes of older embryos, stage nine, e, 

 eye; es, eye stalk; ml, m2, m3, neuromeres of the hind-brain; mbl, mb2, divis- 

 ions of the mid-brain; op, otic pit. 



ings now appears in the hind-brain back of the cerebellum form- 

 ing in order from in front backward, the completion of which is 

 seen in the older embryos B and C. (ml, m2, m3). Between 

 the second and third swelling appears at this time a pigmented 

 crescent shaped depression the otic pit (op). The presence of 

 these various swellings along the neural canal gives a very charac- 

 teristic segmental or beaded appearance. 



This description of the swellings or '^neuromeres" of the 

 closed neural tube agrees essentially with the results of other 

 authors. The agreement will be made apparent by a more 

 detailed discussion of the important features. 



In the embryonic fore-brain of vertebrates Kupffer ('03) 

 distinguishes three regions or, as he calls them, ' ' secondary neuro- 

 meres," the telencephalon, the parencephalon and the synen- 

 cephalon. Locy ('95) and Hill ('00) found three primary neuro- 

 meres in the fore-brain before the development of the secondary 

 divisions but they could not trace the primary divisions into the 

 secondary ones. Neal ('98) considered the fore -brain to be one 

 large neuromere which later showed secondary divisions. The 

 relation of the first procephalic neuromere to the fore-brain 

 has already been discussed. In Amblystoma the telencephalon 

 appears as Kupffer has described it but it develops so late that 

 no satisfactory explanation of its relation to the primitive seg- 

 mented neural axis can be given. Behind the telencephalon 

 lies a division called by Kuffer the parencephalon. The lower 



