^8 McCLURE. [Vol. IV. 



segmented region. He does not, in his brief paper, give any 

 of the histological characteristics of the segments. I am also 

 indebted to this paper for many bibliographical references. 



Gegenbaur has recently expressed the following opinion: 

 " So interessant und so vielversprechend diese Thatsachen sind, 

 so wenig scheinen sie mir gegenwartig geeignet, zur Beurthei- 

 lung der Metamerie des Kopfes selbst als Faktoren in Geltung 

 gebracht zu werden. Das wird erst eintreten konnen, wenn 

 ihre Beziehung zu anderen, den Kopf aiifbauenden Organen 

 erkannt ist." 



In 1887, Orr described six folds in the hind-brain of the lizard, 

 five of which are of equal size, and the 6th, from which the 

 loth nerve originates, somewhat longer than the others. He 

 described the mid-brain as consisting of one fold, and in addi- 

 tion to this described two folds in the primitive fore- 

 brain. He gave the name " neuromeres " to these folds, — a 

 name previously used by Ahlborn with a somewhat different 

 significance. Orr found that the V., VH., VHL, IX., and X. 

 nerves each originated in connection with a neuromere which 

 degenerated after the nerve was formed. He fully described 

 the typical structure of a "neuromere," which I quote, as it 

 bears directly on my own work : 



1. "Each neuromere is separated from its neighbors by an 

 external dorso-ventral constriction, and opposite this an internal 

 sharp dorso-ventral ridge, — so that each neuromere {i.e. one 

 lateral half of each) appears as a small arc of a circle." 



"The constrictions are exactly alike on each side of the 

 brain." 



2. "The elongated cells are placed radially to the inner 

 curved surface of the neuromere." 



3. " The nuclei are generally nearer the outer surface, and 

 approach the inner surface only towards the apex of the ridge." 



4. " On the line between the apex of the internal ridge and 

 the pit of the external depression, the cells of adjoining neuro- 

 meres are crowded together, though the cells of one neuromere 

 do not extend into another neuromere." 



"This definition of adjacent neuromeres presents, in some 

 sections, the appearance of a septum extending from the pit of 

 the external depression to the summit of the internal ridge 



{spt)r 



