The Nervous System of Amblystoma. 431 



The segmentation of the closed tube in the region of the 

 medulla was observed by the early anatomists and embryolo- 

 gists (Balfour, '81). These segments were first carefully studied 

 by Orr ('87) in his work on Anolis. He was the first to use the 

 term neuromere, and his description of a typical neuromere has 

 ever since served as a criterion of a neural segment in the closed 

 neural tube. He says: "Each neuromere is separated from its 

 neighbors by an external dorso-ventral constriction, and opposite 

 this is an internal sharp dorso-ventral ridge — so that each neuro- 

 mere {i.e., one lateral half of each) appears as a small arc of a 

 circle. The constrictions are exactly on each side of the brain. 

 The elongated cells are placed radially on the inner curved surface 

 of the neuromere. The nuclei are generally nearer the outer 

 surface and approach the inner surface only toward the apex of 

 the ridge. On the line between the apex of the internal ridge 

 and the pit of the external depression, the cells of adjoining 

 neuromeres are crowded together, though the cells of one neuro- 

 mere do not extend into another neuromere." He found six 

 neuromeres in the hind-brain. The first and fifth have no 

 nerve connection; the second is connected with the fifth nerve; 

 the third to the sixth nerve; the fourth to the seventh and eighth 

 nerves; the sixth to the ninth nerve. He considers that the 

 mid-brain is a single neuromere and that the thalamencephalon 

 comprises two neuromeres. He does not regard the secondary 

 fore-brain as being a true neuromere. McClure ('90) was the 

 first to claim that these neuromeres of the closed tube extended 

 in a regular series to the anterior tip of the brain. He found that 

 Amblystoma nad one less neuromere in the hind-brain than Orr 

 and Hoffman had found in reptiles. He considered that the 

 the difference was due to a fusion of two neuromeres. He found 

 in the forebrain evidences of two neuromeres and possibly a 

 portion of a third. Waters ('92) confirmed McClure's account 

 of the absence of one neuromere in the hind-brain of Ambly- 

 stoma, but in the teleost brain he found the whole number, in 

 the hind-brain six, and assigned to the mid-brain two, and to 

 the fore-brain three. Kupffer ('03) from the results obtained 

 Dy McClure and Waters concludes that Amblystoma presents 



