The Nervous System of Amblystoma. 479 



similar origin. The relation of the hemispheres to the first 

 neuromere presents a difficulty to which Amblystoma seems to 

 afford no solution. To the second and third neuromeres are 

 added the optic lobes to form the pecuUar sides and roof of the 

 mid-brain, a history which is very clear and definite in Ambly- 

 stoma forming the most valuable contribution of this paper. 

 At the sides of the closing neural tube in the region of the fourth 

 neuromere develops the cerebellar crest. To the history of the 

 modification of the neuromeres of the medulla no facts were dis- 

 covered to add to the account already given by Locy and other 

 authors. 



Fourth. ''The differentiation of sense organs." Two facts 

 have been discovered. The exact position of the anlage of the 

 lateral eye has been shown to be just lateral to the neural plate, 

 not on it. Not only is this anlage incorporated into the neural 

 tube but also its ganglion, the optic lobe, is incorporated to form 

 the roof and sides of the mid-brain. 



Summary 



I. Four grooves appear on the embryonic area of the egg in 

 the following order: the posterior germinal depression, the 

 anterior germinal depression, the blastogroove, the neural groove. 

 The last named alone persists until the neural tube is formed. 

 ■ ^ II. The axis of the nervous system is the neural plate which 

 is divided, longitudinally by the neural groove and is surrounded 

 by the peripheral groove. 



III. The neural crests arise independently of the neural 

 plate, being separated from it by the peripheral groove. 



IV. The anus arises as a posterior prolongation of the blasto- 

 groove. ^ 



V. The anterior end of the neural plate, the procephalic 

 lobes, is marked off from the rest of the plate by a deep transverse 

 groove and also by its darker color, its greater width, and its 

 elevation above the rest of the plate. 



VI. There are four neuromeres in the procephalic lobes. 



